Because the law hasn't caught up to reality yet.
Airbnb has made a significant positive difference in my own life. I would've been in a bad spot if not for the places I found via Airbnb. No idea whether the transactions were legal, but in the end I found a place to live and the host found a tenant. Win-win.
The problem is when people rent these places and then throw loud parties. The neighbors' rights are violated in that case. I don't know of a good solution, but I don't think it's worth sacrificing the entire platform due to that problem.
EDIT: It seems like if people here were given the option of either casting me out into the street or letting me rent an Airbnb apartment, some would honestly and truly choose the former.
In the part of America I was in, you can't get an apartment without proof of employment, unless you have more than $15k in the bank. (Something like a year or two worth of rent. I forget the exact amount.) I was between jobs at the time, and needed a place to stay to get another job. So, which illegal act would you have me do: Get a friend to say they were "employing" me as a consultant, or rent an apartment via Airbnb? Why would you be okay with the former, but not the latter? And if you're okay with neither, then what would you have me do?
There's just a small matter that the law hasn't caught up to reality yet.
The thing that frustrates me is that there are pretty obvious solutions, actually:
1) AirBnB needs to provide a service where anyone can check to see if a building has any AirBnB units in it. Then they need to be able to file complaints about the unit.
2) AirBnB needs to require written permission from the landlord of the property.
1) seems completely feasible to implement for me, and I'm pissed they haven't yet. The only problem is once they have the complaint, they'd pretty much be required to notify the landlord which leads us to 2) which also seems feasible, but would obviously obliterate their market. Ultimately they've overextended themselves on that front, and either they will be successful lobbying change, or a company that figures out how to do what they are doing while making peace with landlords and the hotel lobby will come along.
What isn't legal (here in NYC and many other places) is having someone rent out a completely unlicensed, unregulated, uninspected room as a 'hotel', which is what airbnb is mostly for.
Funnily, airbnb's silly new commercials here in NYC only touch on the folks who rent out a room in their apartment. They don't mention the illegal full-apartment rentals that are the majority of their business at all.
I don't know who cares of a party in a flat. Noisy parties existed before airbnb.
To me, the problem is that since there is no regulation, renting a flat for short periods allows to make much more money than renting it for a long time to a reasonable price. It contributes to the shortage of flats to rent supply, the rise of the rent prices and gentrification.
Parties are one problem. I'd be more fearful of theft and destruction of property... and a general discomfort in having complete strangers coming and going from my building. That said, it seems extremely rare that an Airbnb guest would engage in the kind of behavior that would generate headlines.