You might get surprised - I looked at default KDE, Gnome and docs for a couple WMs. Yet my question is not “how do I get different backgrounds”, but rather, “how come is this the usual thinking”? Why? Are there serious drawbacks technically or from UX perspective that I didnt think of?

Why downvoting? It is good for community to know this, even if content is far from cheerful. Expressing criticism in comments does more for public awareness than making the message less visible.