I mean, object safety is one exact example. Every trait can be monomorphized, but not all traits can be made into an object. So "turn every generic into an object" just doesn't work. When you request a generic bound, you're saying that you want certain performance characteristics.

Haskell's polymorphism is similar, it's true, but I can't speak to their semantics well enough to intelligently comment. They are much less concerned with performance than Rust is, so they tend to do less of the sorts of optimizations that gets Rust into trouble here.