Bad reviews are often moderated away and refund requests are stonewalled (and few people know about chargebacks so that isn't factored in). It is easier for the platform to acquire guests than hosts, so the platform takes the side of hosts.

Edit: at least that was the situation a couple of years ago. A host below now reports that the situation has changed and they take the side of the guests; however, either way it's open to abuse no matter which side they take.

The issue is that in a marketplace where both sides can be dishonest, the only way to ensure quality is to do spot checks by trusted actors (aka company employees) where the penalties for failing such a check are dissuasive enough that it becomes more profitable to play by the rules.

This is similar to how law enforcement is supposed to operate - the reason the penalty for theft (for example) is more than merely returning the stolen items is that since law enforcement can't observe everyone all at once, the penalty needs to be enough of a deterrent to make the bad behavior unprofitable overall, to discourage it even in cases where law enforcement isn't there to witness it and enforce said law.