macOS is at least 20 years old, and Apple needs to maintain it indefinitely. They need engineers who aren't going to accidentally break things for the 100 million strong existing user base. Nobody rapidly specializes in Mac development, there is simply no substitute for experience in this area.

The results are pretty obvious and depressing when you see Apple throw engineers at the Mac who don't understand the Mac. Low quality, bugs, constant accidental breakage, bad design in opposition to the platform standards.

Not every engineer can work on "the next great thing". You need a lot of engineers to work on the old great thing too. A mixture, a balance of engineers is important. A company gets diminishing returns from having all jack-of-all-trades engineers.

Google is the absolute worst at this. They're constantly creating new things... and then discontinuing them a few years later. Their follow through is terrible. Nobody wants to do long-term maintenance. Even though most of Google's revenue comes from the old things, such as search. (My impression that Google search is actually less accurate and useful now than it was 10 years ago.)

People love to make sports analogies, so here's mine: imagine a football team that put 11 all-star quarterbacks on the field. They would be the worst football team in the world. Every team needs role players.