[ghaff]

For starters, lots of them just wouldn't move. When companies move headquarters or other large facilities they usually expect to lose lots of people even if they cover relocation and actively try to hold onto people. (Which may, of course, not be the case if the company actually wants to lay off people.)

None of this is to argue that big tech companies shouldn't be more distributed. They probably should and it's happening to some degree naturally as they grow. But trying to move already employed people wholesale to a new city, especially one that many will see as relatively unattractive, will cost you a significant percentage of your workforce.