I'm not sure why having a screenshot is required but a description is optional. I would think you should be able to set either and that would enable the richer UI.
iOS store
macOS store
Windows store
Steam
Epic
Play store
Discover
(Gnome) Software
All these support screenshots. It's pretty reasonable that after a decade of millions of people installing apps this way they would want to see what they're getting into.
Yeah something generic would help, but manifest should still absolutely support the idea of screenshots.