When you start a new job, do you have to go through the interview process? Do those companies make you bang out FizzBuzz on a whiteboard in front of the team? Or do they just pretty much take the fact that you authored a popular project as an endorsement of your technical skills?
References are generally taken much more seriously at that level as well (i.e. taken from mutual acquaintances who have incentives to be open).
The second reason is cultural fit. You're going to be spending a lot of time with these people, you want to make sure you're going to get along.
As well an interview to me is two way. As much as the company is evaluating wether I am a good fit for the company, I'm evaluating wether the company is a good fit for me. If I am actually good enough that I should be offered the job without an interview then I certainly should be good enough to prove it in an interview.
Also anybody who feels above going though a company's interview process should be a massive red flag as an employer.
If you're a "superstar" (I don't really like that term), you want to work with other talented individuals. So if the interview process is non-existent or overly easy, it sends a negative signal about the "potential" quality of other hires.
Of course, I've gotten ones with a github url with no repos in it or some piddly little thing, and that sends a different signal.
Oftentimes, the great coders are so anal about the code they write, that it's highly refined, well organized and thorough (mostly because they can't attach their name to bad code, especially when it's public). Those are the guys you really want on your team.