Take for instance something like suggestions on form fields: you start typing something and it presents some options from a hardcoded list that matches the prefix. This is natively achieved through the HTML element <datalist>. However, <datalist> implementations on most browsers suck to the point of being unusable.
The drive to roll your own is not so much that "it would be fun to learn this" as much as it's "rolling my own would let me express my vision exactly". What draws a lot of people to software engineering is that it lets you make anything you imagine. This is also what makes a lot of devs turn their nose at no-code and vibe-coding.
If you can't build things exactly how you want them to be, then it's hard/impossible to build something that's truly genius.