multiple reasons - it's hard to make declarative UIs with C# (recent versions try to make it easier, but it's still not there) - making UIs is hard, making ga

multiple reasons - it's hard to make declarative UIs with C# (recent versions try to make it easier, but it's still not there) - making UIs is hard, making game UIs is even more hard because there is no rule to make a UI, it depends on many factors (art style, game genre, platform you target, input you support) - making something customizable, serializable and performant is very hard you are better making your own that suits your need, hence people rely on 3rd party solutions that exactly solve their use case