Because inviting total strangers to inhabit your home while you are away is such a great idea.
I rent from there quite frequently. I rented three places in Panama, three places last year in the Czech Republic and a place next month for three weeks in Florence. I really love it, especially in foreign cities that are expensive. I rented a place for a month in Stockholm for about the price of five nights in a hotel there.
I always leave the place cleaner than I found it.
It's also like any other site, you get reviews as a renter or a rentee.