The browser is not an appropriate medium for games that need non trivial access to local storage and system input. Browser compatibility, and being subordinate to multiple third parties isn't great, either. Rather than fighting with browser limitations, it's easier to package, deliver, and maintain a stand-alone application.
Steam isn't a major consideration once you've gone into the weeds of implementing software that colors outside the usual browser lines.
That security advantage is the reason it's inappropriate for certain things. One of those things is i/o intercept. You don't want browsers to be able to do certain things that games or first class apps can.
Steam isn't a major consideration once you've gone into the weeds of implementing software that colors outside the usual browser lines.