It's just down to tradeoffs on what you're looking for. You can get a kitchen and a bit more space with an airbnb, but you give up on the hotel amenities of support/cleaning staff, and lounge/gym spaces. Don't get me wrong, I usually value a full kitchen over maid service. But I was able to stay at nobu for a comparable price to a 2 bedroom in Barcelona.
I do agree with your point about renting a room in someones place. But that's now a small fraction of airbnb's business compared to renting a full place.