Which is kind of insane, right? The whole point of the bidding system is that it ends in an optimal position, where nobody would pay more for a room than someone else is already paying. Whereas with an arbitrary system, you might well end up in that situation.
I suppose the insight is that (some or many) people are more ready to accept an unfair situation dictated by a dispassionate external authority (even if that authority is just some JavaScript!) than a fair situation reached by negotiation.
Part of what might make this reasonable is that this process takes who is moving in together as a given. Say you and I want to be roommates, and you happen to know that I have a pathological fear of sleeping in big bedrooms. Clearly the efficient thing to happen if you're a normal person is for you to get the large room and me to get the small room. But how much do we pay?
It seems rather unfair of you to incorporate that information into the bidding system and bid up the price of the small room (a sort of strategic bidding). But it also seems unfair that if we both answer honestly the system has a good chance of making the smaller room priced the same (or more than) the big room.
With any other roommate, especially one who didn't know about my pathological fear, I could expect to pay less for the smaller room. These "external authorities" might reflect such widespread notions, as well as avoid the risk of strategic bidding that can occur after roommate selection is fixed.
I suppose the insight is that (some or many) people are more ready to accept an unfair situation dictated by a dispassionate external authority (even if that authority is just some JavaScript!) than a fair situation reached by negotiation.