Why? Those kinds of game engines are enormous amounts of code, and there's little incentive to rewrite.

I do strongly disagree that we aren't ever going to see large-scale game development in Rust; it just takes time. Whether games adopt an engine is largely about that engine's maturity rather than anything about the language. Bevy is quite young; 0.13 doesn't even have support for animation blending yet (I landed that for 0.14).