We have so many custom UI Elements, because forms did and still do suck. RSS always had a back seat, because it always was second class in browsers. Semantic web died outside of search engines, because browsers did not offer features like "Save event to my calendar". And boy, have I been promised for a long time I might someday be able to sort HTML tables by click.
Just a non exhaustive list of my pet peeves, but there are many more examples.