I agree, there are very few really good IDEs and the majority of them are closed source. The only open source one I can think of off the top of my head is Kdevelop, and last time I tried it it was not great.

That being said, I think the reason for that is that most FOSS projects are stuff someone started and maintained because they wanted an alternative with XYZ, and for IDEs a good chunk of people who could build excellent IDEs don’t even use one, so they don’t even start to work on it. The reason is that vim/emacs are so great it’s very hard to beat them, I think a good configured vim/emacs can beat anything the best IDEs can do, and while configuring vim/emacs to get to that level is difficult, it’s stile much more easy than building an IDE from scratch. So you’re left with a gap where beginners don’t have any tools because experts don’t need them.