Because browsers follow a pattern of reinventing the wheel with different tradeoffs, primarily focusing on "secure" execution of untrusted code from the web. Ju

Because browsers follow a pattern of reinventing the wheel with different tradeoffs, primarily focusing on "secure" execution of untrusted code from the web. Just look at the stack of web technologies, they provide an ever-expanding set of abstractions and reinventions of what OSes already provide.