It's not just pay. Things like pensions that would encourage long tenures have been all but eliminated from compensation packages. The idea of staying at a job for more than 3 years, especially in IT, is crazy to people. If you're there for >5 years and then look for something else, interviewers wonder if something is wrong with you.

Which is insane. Companies lose a lot of value by not having long tenured "company [wo]men" anymore. I keep waiting for some convoluted explanation that shows this situation is better in even a strictly capitalist sense, but that explanation doesn't seem to exist. The best I have is that people coming from outside organizations will cross-pollinate ideas and technologies instead of being stuck with whatever that particular company is doing. But there are other ways to handle that, and you don't have to push it on everyone.

No, companies just seem to have decided this is how they're going to operate.