> Can you imagine if every button on every site was the same Windows button gray color, regardless of the site's color?

Not a webdev, but can't you just use CSS on the <button> element for that?

Yes you can, on a small/simple site. But on a serious web application sticking to plain HTML/CSS will be far too limiting, in many ways.

There's a reason why 99.9% of web apps use JavaScript, and with it a tool (framework) like React, Astro, Angular, or Vue. And if you're using such tools, you use them (eg. you use React "components") to create a consistent UI across the site.

But again, which tool you use to develop a site has very little to do with what design choices you make. A React dev with no designer to guide him might pick the most popular date picker component for React, and have the React community influence design that way, but ... A) if everyone picks the most popular tool, it becomes more idiomatic (it's not doing this that creates divergence), and B) if there is a human designer, they can pick from 20+ date picker libraries AND they can ask the dev team to further customize them.

It's designers (or developers playing at being designers) that result in wacky new UI that's not idiomatic. It has (almost) nothing to do with React and that layer of tooling, and if anything those tools lead to more idiomatic design.