Arbitrary predictions markets sound good in theory until you realize that, as the market grows, eventually you reach a point where there is an irresistible financial incentive for the market participants to intervene in the outcome of the event in order to claim their reward. The most egregious case is "will so-and-so be dead by such-and-such date", which is plainly just an assassination market in disguise.

There are defi prediction markets. As other commenter said big issue is that it leads to fixing. One of the markets was something like "number of tweets this celebrity will have" and eventually someone paid them to start deleting tweets.