I think it's pretty simple - sellers pay to list properties via agents. Agents pay Zillow for leads. None of those revenue streams would benefit from hosting free/comments from randos.
Also, if the comments are anonymous... it seems inevitable that neighbors, people wanting to buy at a discount, sellers, etc. would post misleading garbage on there to further their own interests. It's pretty much inevitable that any information rom third parties would be dubious garbage. e.g. "this house has a leaky roof/mice in the basement/mold in the bathroom..." well, sure, most housed do at one point at another, doesn't mean it wasn't fixed/already priced in etc.
Is that just a way of saying "houses that doubled in price the last few years"? Or equivalently "houses that were half the current asking price only a few years ago"?