well it's historically been because 'extend the native widget' isn't possible with css. WebComponents change that a little bit with well-scoped css, but that's still a very javascripty solution.
Of course, I’m not at all advocating that the solution is to just do it but that web really hasn’t kept up with the needs of the kinda of apps people want to build with it. It’s easy to build something that works but annoying to impossible to build something robust so we’re left with every single website being functional enough to work but broken.
I don’t think telling developers “just do it right” is a scalable solution, the platform has to make the least effort path the one that works best.
I don’t think telling developers “just do it right” is a scalable solution, the platform has to make the least effort path the one that works best.