This seems like more of a complaint about egui's choice to re-implement all the widgets from scratch than its choice to use an immediate mode API. The two are orthogonal. The IMGUI idea has its roots in a community of people that like implementing things from scratch, but you could implement it on top of native widgets if you wanted to.

The choice to re-implement native widgets from scratch is justifiable if the egui devs think they can do better than the native widgets do. It's also nearly required in practice if they want the toolkit to have consistent behavior across multiple platforms (something people have learned the hard way multiple times across multiple different UI libraries).

It's hard to get it right, but that's why the egui devs are writing a library: so that others can benefit from that hard work. Many of the things you pointed out are just bugs, and bugs can be fixed.