In a low-fraud world, it's probably cheaper for airbnb to insure against it themselves than to outsource that to somebody else. You don't insure against a large number of small losses, you insure against the possibility of one big loss -- this is why you buy car insurance but Avis doesn't.
At least in the short-term there is one big risk for them: They don't know how many users will claim that there have been damages. So they don't really know how much this insurance will cost them.
By outsourcing the insurance to an insurance company it's not their problem. It's the task of the insurance company to assess the risk and Airbnb knows beforehand how much it has to pay.
In a low-fraud world, it's probably cheaper for airbnb to insure against it themselves than to outsource that to somebody else. You don't insure against a large number of small losses, you insure against the possibility of one big loss -- this is why you buy car insurance but Avis doesn't.