Would you have asked this question for a game you bought on Steam? Because the answer to your question is basically the same.
But since there is no real difference between a publisher/developer account and a user account on Itch, there is the asset section aimed at creators and there you will find things meant to be used in other projects. Those projects do include licenses or at least spell out what you can do or not do with the asset. Personal usage for games does not need a license, but using a thing in a project you want to distribute needs clear rules - mostly because it is not allowed per default. Not seeing a license does not mean anything goes!
The web games you see here are not assets. If you can buy them, you are not buying the web game. You are buying whatever download version of the game there is. If there are no downloads, a web game cannot be a paid game, it can only collect "donations".