[ximm]

That is a very broad statement. What specifically do you find terrible?

Here is a list of issues I often have with JS-based alternatives that do not exist with alert/confirm/onbeforeunload:

  - the escape key does not close the modal
  - tab focus is not restricted to the modal
  - the modal is not properly announced by screen readers
  - the positions of confirm and cancel buttons differ across sites, leading to misclicks
  - the modal is not or very hard to use on mobile

Building good modal dialogs is hard and much easier done in the browser than on the page. Even <dialog> (if it ever becomes a reality) will not solve all of these issues reliably.

So in a way I agree with you: there might be more user friendly solutions. But the average alternative that people come up with will be worse, not better.