Steam doesn't need to be ran with elevated privileges in order to operate properly though, whereas X-Sim does.
It's the elevated privileges which makes it a bigger issue than other applications with built-in browsers.
With regards to steam, Valve also has full control of the pages it displays within the steam browser. If you attempt to load an external site you'll get a warning that you're leaving the Steam/Valve 'walled garden' and the link will be opened in your regular browser, not in steam. This wasn't always the case, but was added to prevent phishing based attacks used to harvest user accounts.
Steam has also had
far more serious browser-based vulnerabilities in the past, so they're not immune just because they're a big company.
MotionJoy is another similar application which requires elevated privileges, but what's worse is that it also defaults to loading web ads from shady Chinese * sites. They've allegedly had issues in the past with these sites taking advantage of the browser's elevated security permissions to install malware/viruses. Some people even think that the application itself is a scam designed to infect the user's computer. I wouldn't go that far, but running a web browser with elevated permissions is pretty much always a really bad idea.