Because localbitcoins.com it self don't have the control over the seller on their site. The seller has the authority to change the price which the seller could take advantage of that. That's the bad thing localbitcoins. Some sellers are abusive at their pricing. They price too much compares to exchanges regular price. Others are scammers. So be careful dealing with sellers there. Make sure you check everything before you make order to avoid mistakes which cause you losses. I saw a user here bought $200+ worth of Bitcoin for $875. That's not even on it's half.
At the end of the day if people decide to accept a bad deal that is their fault, this is no different than to look for the best prices for any product, you need to look at different providers and choose the one that has a good combination of reputation and good price, after all bitcoin is about establishing a true free market and if people want to charge a fortune for their bitcoin they can do it.