The seats are assigned, the baggage compartments are self-serve.

Limited overhead space. If you wait, you increase the chance there won't be room for your overhead bag.

Overhead bin space for carry-on bags.

As someone who travels with carry-on only (as much as possible), if I'm in economy I make sure to board as soon as I can to ensure there's space in the overhead bins for my bag.

Two reasons:

I always take window seats, and don't really want to hop over someone to get to my seat.

And it also means I don't have to fight for overhead space for my carry-on.

Surprised no one mentioned this, but carry-on bags are no longer free for all - only free for the first ones in line.

Anxiety (especially now that planes are oversold and standby passengers are nearby waiting to grab empty seats...), the need for overhead bin space, not wanting to have to climb over people, illogical impatience, etc.

These same people will all stand up at once the moment the plane stops.. and continue to uncomfortably stand there for like twenty minutes.

On my last flight my gf sprung up and was irritated that I wasn't doing the same. Like where the fuck you going to go? One foot to the left? Just, why? I'm chillin.

If you have a bag to put in the overhead compartment then you want to get on sooner since the space is limited. I personally only ever take a bag that can go under the seat. I wait until everyone has boarded regardless of my group.

I'm hard of hearing and terrified of standing in the wrong place at an airport and missing the visual cues to board the flight. Once boarding starts and people start queueing up, I usually get in line because it's helpful to see what everyone in front of me is doing - the order that they hand over paperwork or get carry on double checked. I can't guarantee I'll be able to hear the attendant if they ask me questions at the gate because it's so noisy, so I like to at least feel like I'm prepared.

One time I was flying with crutches and qualified for early pre-boarding because I needed the plane wheelchair (skychair). I sat right next to the gate desk and waited, then I started seeing people queue up so I quickly joined the line, wondering how pre-boarding works when the whole plane of passengers are already vying to be at the front of the line.

I get to the front, the attendant looks at my ticket then after some awkward back and forward eventually I realised they were telling me I'll have to wait till everyone has boarded to get the sky-chair on. I should have come to the desk when pre boarding was announced. I pointed that I was sitting right in front of them... Apparently they were called my name 3 times over the loudspeaker.

Apparently airports can only comprehend one disability at a time (if that!) they knew I was hard of hearing (it's on my ticket) but still thought calling me over the PA was the best way to get the attention of the deaf person sitting 80cm from their desk.

So I sat back down and waited for the line to clear, then I got back up when there were 2 people in line, and after another back and forward I learned that they had tried calling my name again about halfway through boarding because they only had one skychair and it was now or never because the chair had told fly with the other passenger because their arrival airport didn't have a chair, or something, I dunno, anyway I kind of had to crawl down the ailse to get to my chair because in the past I've just used the backs of chairs to swing myself along, but the plane was full so I couldn't do that.

Overhead space. The only upgrade I pay for is boarding group, and I want to maximize my upgrade purchase, so I want to be at the front of group 2.

I only travel with an appropriately sized carry-on but I'm a bit tall so I can NOT have the bag by my feet.

So I want to guarantee I have the overhead right above my seat.

I also generally work on the plane so I can get my laptop and such out easily while standing at my seat, then put my bag up worry free.

Yup, exactly what I think. So I used to sit in the gate area until the last possible moment until I found out that if you wait too long, they'll give away your seat. Plus since they are charging people for checking luggage through, you have to get that overhead bin before it's gone, unless your carry-on fits in front of you, which it doesn't if you're on a bulkhead. So, now I stand in line, frustrated by that and all the other crap we have to go through when traveling by plane these days. Looking at you TSA.

People are posting a lot of maybe more rational reasons, but I think there's another answer that's more in line with just being a human. Airports suck, air travel, generally, sucks and the whole process is riddle with both intentional and also just unavoidable misery. Every time a new step in the sequence of unpleasant and boring steps that is air travel nears, we start to anticipate it and get anxious to move on to that next step in the process. It doesn't make it faster, it likely only makes the misery arguably worse, but some times people just can't help trying to mentally hasten things even if in reality nothing is hastened at all.

The sooner I get to my seat the sooner I'm out of the god damned airport and can begin whatever escapism I've chosen for the flight. The thing I truly hate is the airport and being in it.

For me it's the last chance to stand for a while. If I'm going to be sitting in a cramped seat for 2+ hours then you bet I'm going to be standing before hand. So the if I'm standing then I might as well be in line.

I’m about to be seated for five hours straight with no possibility of standing or moving around. I’m standing while I still have the opportunity.

Stand, don’t stand, I don’t care. But I’m sitting in the marked disabled seats, with a gate checked walker, and only similarly disabled people or people wrangling little kids are getting on before me, so standing in front of me only tempts me to run into the back of your legs with my walker to bulldoze you out of my way. Leave those of us with extra trouble moving a little space, please!

Overhead space for carry-on bags.

I didn't used too get in line. But I've had a number of times someone took to much carry on (how I don't know) and took all of the overhead space, refused to fix it, and the airline didn't give a fuck either.

I rarely sit when waiting for a plane (at least not for maybe an hour before). I'm about to sit for hours, so I don't want to start now.

Because you do not have assigned overhead bag storage, and people bring either bags that are too big, or bring multiple bags

Why does it seem like everyone in the comment section takes flights all the time? For me, flying on an airplane is for vacations abroad only, and I maybe get 2 flights a year at most. I assume this is one of those US things I'm too Asian to understand.

Standing in line brings me joy

I'm about to sit down for the next six hours. I'm happy to stand for a bit.

CGP Grey has a good video on a better way to board planes.

Even worse, people have to pay to be in an earlier boarding group. I think they use limited overhead bin space as a hostage.