Something there is that doesn't love a wall . . .
Maybe. But it wouldn't be paint. There's everything, in fact, about a blank wall that seems to require touching up, decoration, even defacing. The urge to make one's mark in as prominent a way possible seems to be a human trait, evident from the earliest times.
How much better if the one with the urge happens to have some talent. There's a wall painting going on not far from where I live that's a part of a Giant Storybook Project, a collaboration between an art couple, Herakut, and Jim Carrey (who, surprisingly, is something of a wall artist himself). And watching the woman wield a roller on a long stick high up in a cherry picker, I'm impressed: a stroke here, a swipe there, and the basics emerge — details to be filled in later with dozens and dozens of spray cans and shaping devices.
Of course, your sense of what looks good on a wall may not be the same as that of your neighbour, which can lead to a "It's my wall" "Yeah, but it's my eye" dispute. Fortunately in this case it's my mechanic who'll be looking across at this work, and he's too busy working on my car to have time for art appreciation — or depreciation. But the whole matter of who can paint what on which walls is a little more vexed than you might first assume, raising issues about freedom of speech (is "art" always "speech"?), social propriety, private property, property values and all sorts of other bourgeois concerns. It so happens that the same Jim Carrey was made to take down some of his wall art done on the front of his studio in (big city) New York because it offended some bylaw or other — don't know which or what value it was supporting.
(I once was required to look for years at a very blue, oddly blue, wrongly blue house-front, which led me to suggest, to no one's approval, that the person opposite your house should be the one with the right to choose the paint colour, if paint there was to be.)
So most of us risk averse, complaisant folk take our art indoors, where, too, there's a long history of decorating walls. Only now we hang mini-walls on our walls and call them paintings, and in that way make our art portable. And changeable — a mural being too much like a tattoo (wall art for people without walls?) in that it tends to stick around for a long while, and we have short "sell by" dates nowadays. Perhaps the only thing in the modern house that approximates a fresco is the fridge, which is where the kids' art gets displayed, often in lovely undisciplined and overlapping layers.
And that suggests another reason why we don't decorate our walls, whether outside or in: we have suppressed that human urge to expression and lack the courage and the sense of freedom that children have (we hope, incongruously) to make marks — and then proudly make more.