You aren't looking at the entire picture. Some have trouble in interviews because of learning disabilities and social anxiety but do well in the workplace once they are comfortable with the people.

Add to that the fact that most tech interviews are held in non-real world situations and it's easy to see why some people have problems. Putting a person in a room with a piece of paper with a chunk of code that has a 'hidden' flaw with no access to any other resources isn't real-world.

All of us have access to books, manuals, the code, google, any number of places we can use to help.

And usually these interviews focus on nuances of a language or a programming trick that you only know if you've seen it before or encounter it regularly.

The codebase I'm working on now targets the iPad. If I were to interview with a company and the tech reviewer asked me about an IE specific hack chances are it would take me a while to come up with it. Assuming I've encountered it before.