The APIs are expensive for storing state (read access on attributes) and expensive to write (so many ways to trigger reflows and paints) without a good way to batch them.

WCs have tons of boilerplate to them, such as extra song and dance to make attributes observable. In fact, a custom element can have an attribute and a property with the same name, and they are managed separately unless you explicitly set them up to sync. It's a terrible design.

Built-in things aren't extensible - there's no way to have a number type input that doesn't suck without doing it yourself, and then you need to opt into extra song and dance for your element to participate in form submission data.

There's a reason popular frameworks are popular. They're good enough, you can hire people who already know them, and you benefit from the maintainers fixing bugs for you.