At a guess because there is over all very little need for 'game developers' at AAA studios. Had a friend that worked a AAA studio and he spent most of his time just working on improving and fixing collision detection performance in their game engine. Had another friend who only worked on the asset management pipeline. Another wrote tools for mesh optimisation. Others I know just worked on various tasks related to writing high performance low latency server code. All of these are skill you're more likely to learn during a CS degree rather than as a self taught indie game developer.

Nobody I know who has gone to a large AAA studio has actually worked on game mechanics or game play in any meaningful way. One poor sod I know, who loved computer games, got tasked with working on the framework for building in-game microtransaction stores into their games.