A lot of users don't understand them, ID editor's support of them is minimal, so a lot of users accidentally break them, and they don't even understand what is the problem, because they can't see them in ID.
Fixing them is not easy, and time consuming. There is no easy way to batch edit them, so if someone messes up a roundabout, you have to fix all bus lines one by one. OSM Relatify helps somewhat with that, but it has its own limitations, and you still have to edit them separately.
same for hiking and biking routes. I found some obvious errors but I am not able to fix it with ID. I created some notes for them but since several monthes nobody fixed it.