They don't want to do that because they've already spent a bunch of money on their fancy HQ and don't want to see it empty out.
There are plenty of employers with < 1000 employees, however, crowding these downtown areas. They have way more flexibility in being able to move out of these city centers and into more affordable locations for everyone. They don't because part of the reason for their offices in these downtown location is rich people showing off to other rich people. You gotta "look" successful.