Not going to defend Polymarket, but that's usually the case for markets that should never existed that depend heavily on opinion and/or reporting.

For example, "Will Trump talk Macron before 31 of March?". Someone is going to get scammed no matter what: the market rules will set some basic resolution scenarios, but you'll find sources saying it was a letter, or sources several countries and in several languages saying that it was an exchange between their offices/assistants, or smoke signals, or god knows what, while a single US source will say they talked without any further detail, or it is simply reported 1 month after the fact when the money has long moved wallets, and then people vote based on the combination of factoids they've been exposed to. In principle the perverse incentive of voting wrong to earn money will make you lose money on the UMA itself (unless there's a coordinated effort, which has already happened, but IME it's rather rare).

Other markets are indisputable bar black swan events, like, "how many Oscars will X win", if it's 6 it's 6 and no source will say 5 or 7. That makes opening disputes 100% a money-losing option.