In addition to fba827's point about really bad GMs not having many players available and great GMs having as many as they would want, which I agree with, there is also an assortational reason why everyone's GM may be above average: tastes vary, and part of being a good GM is catering well to your players' tastes. People who like to play in a sandbox will tend to find sandbox GMs, and will then view them as better than average because they are sandbox GMs, unlike those lousy adventure path GMs. People who like adventure paths will tend to find adventure path GMs, and will then view their GMs as better than average because they run adventure paths with strong plots and well-balanced encounters, unlike those lousy sandbox GMs. And people who like high politics and intrigue games will tend to find high politics and intrigue GMs.
The sorting effect won't be perfect. And that's not all that goes into whether a GM is good: I've played with GMs who run my preferred style of game badly and viewed them as poor GMs, and I've played with GMs who run styles pretty far from my preferred style really well and enjoyed playing with them and viewed them as great GMs. But it's an effect that is different from general Lake Woebegone effects that contributes to the "average" GM being "above average."