[XFCE, Openbox] X11: Black screen on session start

System: Nvidia RTX 4070
Pop OS 24.04, latest updates

Steps to reproduce:

1. Install XFCE or OpenBox
2. Try to login using cosmic-greeter as a display manager

Actual result:
Unable. Black screen - doesn't work

How I "fixed" it for myself:
I've switched to lightdm. All worked immediately.

I guess... X11 sessions are not supported at all? Why no indication of that?