AFAIK you just add your instance to the list via git pull request, and then it's sorted based on activity. You cna absolutely screen instances, by reviewing pull requests.

But that takes labour, and people are also pushing for the devs to work on the actual server and client softwares, and their instance is overloaded, and...

This is what happens to small projects, whether they be software or anything else, when they suddenly get a lot of attention. Pace accelerates, but the amount of available labour hours doesn't increase at nearly the same rate.