Not just that, but you have to do physics and other calculations on each object and even between objects many times per second. So if your server handles 20 ticks per second, you only have a budget of 50ms per tick. Even on a beefy, many-core server, that's a lot to do in not a lot of time, and you can't afford to have any inconsistencies with performance (like GC stalling for several ticks).