Does anyone on here know why it is that Unity doesn't just run on .NET? The cross-platform benefits of Mono have disappeared a decade ago or something like that.

Like, why would Unity invest in hacks like il2cpp rather than just port over to modern .NET? Is there some licensing thing going on?

Sunk cost fallacy will be a big factor. They already invested a lot of money/time into customizing Mono and hacks like Burst and IL2CPP, so there's momentum to "stay the course" and continue investing in those. Even if some evidence suggests that it is the wrong course.

The article touches on it - assuming it's correct, it's because they did a bunch of proprietary modifying on Mono, so it's not "just" Mono any more, it's Mono and a decade or more of the most exciting layer violating crimes you can imagine.

I would assume, if someone Meta scale had an app that had to be built on Unity for whatever reason, we'd see similar blogposts to the ones they did about finding bespoke optimizations for their Android app to be performant with how enormous it became, probably involving either some bespoke alternative to IL2CPP, or writing a bunch of bespoke optimizations onto a bastard descendant of whatever version of Mono Unity originally derives from and so supports...or just paying Unity so much money they get to do whatever they want to the underlying source.