Sandboxing.
The less a malware does, the harder it is to detect. Even a simple sandbox will protect your browser data. So in theory the malwares that do hard stuff might get cought in the scanners. And the data stealers will not have the rights to read your files, since you run them in a sandbox.
One might wonder why the Itch app provides a sandbox mode. One can really wonder why.
(My guess is that it was just trivial to implement and sandboxing will also protect the system against shenanigans from inexperienced developers that unintentionally do bad things on the system.)
But checking the shiny new thing carefully is a good idea anyways. If you see a popular paid game for free here, that's suspicous.