Another factor is cost - you're in America, I see. I'm in the UK and routinely pay £10/m for fabric, but I can get something much cheaper for backing wallhangings. I really can't afford to waste good fabric on a backing which no one will see. Nor do I usually buy large quantities of fabric at a time, I generally get quarter metre cuts.
I do get the "but I know it's there" thing, though, as I'm always careful that the backs of my quilts look neat even when no one will see them.