The game console business model is an historical anomaly. In the beginning, all console games were produced in-house by the console manufacturers. The consoles weren't "platforms" at all. However, 4 Atari programmers got wind that the games they wrote made tens of $millions for Atari while the programmers were paid only a relatively small salary. When Atari management refused to give the programmers a cut, they left and formed Activision. Thus Activision became the original third-party console game development company.
Atari then sued Activision for theft of trade secrets, because the Activision founders were all former Atari programmers. The case was settled, with Atari getting a cut of Activision’s revenue but otherwise allowing Activision to continue developing console games. I suspect this was because the 4 programmers were considered irreplaceable to Atari (albeit too late, after they already quit).