Yes, there are plenty of great sub-20 minute matches. But there are far, far more great matches that go longer than 20 (I'd say between 20 and 30 minutes is the sweet spot). Not only that, the overall quality of those longer matches is far higher. I'm confident that if you made a list of the 20 greatest matches of all time that go longer than 20 minutes and one of the 20 greatest sub-20 minute matches, the median quality of the former list would absolutely smoke that of the latter. The fact that most wrestlers are incapable of reaching those heights is immaterial. That's like penalizing Metallica because most guitarists can't play Kirk Hammett's solos.