Browsers are a massive improvement as a platform for developing code, or at least they have the potential to be.
Otherwise mobile operating systems have had isolation and a robust native platform for years. It's kinda why mobile apps absolutely exploded and browsers have been desperately trying to keep up for almost a decade now.
There should be a FOSS OS with powerful OS-level security features, but it just hasn't happened.
This is totally different from the actual capabilities security model, which is closer to a "if you can name a resource you can access that resource". So, for example, imagine every file on your system has a path with a uuid, and there is no "ls" - you'd only be able to access files whose names had been told to you/ that you created yourself.
That's a capabilities model, and Linux does not have that.