Because, in addition to the other valid points raised, modern "Anti"-Virus Software is often worse than an actual Virus.

There are way too many pop ups, the menus are confusing and constantly try to upsell you. If you want to remove the damn thing usually it doesn't work, or doesn't work completely, or has a separate auto-updater that reinstalls it after the next boot.

False positives screw you over good (Kaspersky killed the Ethernet Network on a buddy's PC. He couldn't use the internet on it until he managed to remove that piece of shit from his system completey) and are not less frequent than with Windows Defender but certainly more annoying (see above example)

If you paid a subscription getting rid of that is a pain as well (BitDefender tried to scam me out of 130€ by sending the billing notif to an email address they shouldn't even have anymore)

Not all of them are shit like that but most are so sticking with the preinstalled Windows Defender that does 95% of the alternatives results in users having a better experience.