For years I thought of doing it. Can’t be that hard. You can imagine how every component would work. You just need a few tables, right?
But it turns out a polished forum that people want to spend time on has infinite polish. Every feature explodes into a fractal of micro polish. You could spend your whole life improving it and handling rough edges and making it nicer to use.
The WYSIWYG editor being a good example. You could work on just that full-time and never run out of things to do. Or the daylight between a MVP notification system and a mature one that sends PM/email notifs, tracks high water marks, lets users mute certain threads, infinite polish.
That being said, I probably will embark on a custom form just because I'm highly opinionated and capable.