What do you mean? There's plenty of simple standard HTML to achieve nearly everything needed (except fancy nonsense and tracking). As GP says, simple, highly accessible, very performant, easy to understand (both for the developer and the user).
For developers supporting projects implemented using simple raw-js or things like jquery, it's no wonder they developed a habit of DIY. I've had review so much code and push back on this stuff, so I fully understand why it happens.
God, that was such a pain. I recently ripped 20k(!) SLOC out of a project that was finally allowed to drop support for those old Safari versions. All just to display one single datetime picker.
The entire rest of the project is less than 2k SLOC.
I hate when people try to do the dropdown and the "write your own" in one field. It's very confusing how to use those.