I'm going to go out on a limb and offer up the usual and customary response for "home-made" whether it's baked goods, clothing, or whatever other "home-made" item: "I can make it for less than that."
Given this mentality, they (general public) probably ASSUME that quilts also fall into this category- that the prices they see in stores equates to MORE than what it can be made for- not realizing that in recent decades, global marketing has flipped the tables and nullified the entire statement.
And on an unrelated reasoning, historically, quilting was done in the home with material that was recycled from whatever means...perhaps (more often than not) there was no cost for the materials. And since women did this at home (ie: not "work-for-pay"), there was no out-of-pocket cost for quilts. So this may also explain that since there was no "cash" outlay for quilts, people have never actually "valued" them in any sense.
And then there's my "Anne of Green Gables" reasoning. Remember the scene where Matthew wanted to buy Anne the blue dress with the puff sleeves? It was a "store bought" dress. "Store bought" was special, NOT the home-made item.