Well, those hotel brands you cite are all legal businesses. Airbnb is illegally operated in most of its large markets. The illegal risk is currently put on the backs of its hosts, but that could easily change (either way--there could be a legal breakthrough with upside for Airbnb, but it's a municipality by municipality grind).

That makes the value quite hard to pin down, a bad court case or two could severely harm Airbnb. It's not likely that there will be a court case that makes tax paying and code abiding hotels illegal, which makes hotel chains much easier to value.