LOL, there are no set criteria or guidebook for how or when any company should be able to purchase a game development studio. History or not, money talks.
Not saying there is, just elaborating the differences in culture in how both companies approach acquiring talent. Sony's is a very organic way of doing business. They have a long history of making/funding games from independent studios over long periods of time and then offering a partnership that leads to acquisitions. Microsoft literally just whipped out their checkbook and bought/created studios with the biggest names in their fields. They had a long standing relationship with Epic and honestly i feel buying epic would have been the next move for them to keep making games with the way their relationship was. But sounds like there was some spilt milk a while back that soured any idea of that notion. Like during Cliffy B days.