Polymarket is centralized, but one benefit of it is that you can sign up and fund your account anonymously (though you do require an email). Users are not restricted by state or national laws.

On a true decentralized gambling app, email would not be needed, just wallet. It would also be difficult to regulate (the team behind it could be anonymous).

And I'm not saying this is great or anything, but it shows how you can build something with a blockchain that you could not otherwise.

Because of the blackmarket nature of the betting, which is illegal in many countries.