I feel like it’s a historical accident. In the past the platform couldn’t do it all, and if you wanted a dialog you had to use a library. Developers were trained to reach for libraries when they needed something like a dialog.

Then react came along and all developed learning web development after react had little to no knowledge of the platform. They were taught that touching the DOM was a bad thing to do.

That’s about it. Even now when I advocate for vanilla css I get the side eye and “tailwind and shadcn should be the default”. I get it, it’s what most people are familiar with, even if it’s worse than the platform.