People need way more validations than html5 elements can provide.
They need CC number validation. Validations that depend on the content of sibling elements. And they need it to display in custom ways like internationalized or whatever.
The platform provides enough to do forms. It's one of the few things the web is good at. It has been around for ages and works. I mean, what I'm using to type this is a plain HTML form. It doesn't need a massive JavaScript framework that heats up my CPU on every keystroke.