You couldn't even use input type="date" until a couple of years ago because of Safari support. There was more than enough time of the web missing absolutely
basic crap like this that you had to assume that most things weren't available batteries-included, and because of poor modularization related to css and the massive amount of Javascript needed to get anything done, too often the solution was "use a giant-ass framework".
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.