As for 4) probably because it's a high risk and high overhead for most complexes. How do you see that working out? FWIW I often sign short leases (6 to 12 months) and friends who have gone for internships without provided housing have used airbnb. Seems to be a fine alternative.
In my area, I've tried very hard to negotiate leases of less than a year-- they just don't do it, because of our East Coast version of NIMBYism.
Airbnb can be of dodgy quality sometimes, but it has filled most of the demand as you say. There are lots of property taxes and squatter-friendly laws in my area, so I guess thats why landlords feel the need to have very restrictive terms :/
