Conjecture. AirBnB's are generally below market rate for hotels which brings more people in than you'd have otherwise gotten.
Once fees are included, AirBnB is usually more expensive in markets like SF, LA, or NY for individual/couple listings than a comparably furnished hotel room (inclusive of fees).
It gets even better when you split the cost with people. I can get a full, nice house in a great location in LA for a little over $200 a night all in and split it with 5 friends. I would have to get a hotel penthouse for the experience to be even close.
And yes, the fact that it's so cheap does significantly increase the appetite for weekend trips for people my age. The housing part of the trip goes from a major expense to almost an afterthought.