Because commoners know what clothes are and have at one point in their life seen and maybe even handled horses or at least an oxen. They know that nobles have the better stuff, but those things are still mundane.They just take all those freely available warlock pacts then! Or they all become druids, because why not? I'm sure mastery of nature magic would be pretty helpful in farming!
So you suggest some bizarre dystopia where the knowledge of the true origins of magic is supressed? This doesn't seem plausible in the long run.
Also, commoners don't have access to a lot of things the nobles have. So why would they be any more impressed by magic than they would be by fancy clothes, mastery of sophisticated poetry or warhorse and armour?
But do commoners know about magic and that everyone can learn it? Or would they be in awe when a noble does something that is completely impossible for them and might only know something similar from that one priest they heard about (or if they are lucky seen themselves) who is directly empowered by the gods?