Because dedication often does not necessarily equal better results. I hate to say it, especially as someone who is dedicated, but there are numerous upon numerous examples of athletes who dedicated everything they had and were pretty much the definition of a freak and someone else came along and beat them with minimal dedication. There are numerous examples of people who were dedicated and had careers riddled with injuries. Being dedicated really is a relative term because Powell and Bolt still stay out late partying, still drink, sleep through practices, sometimes even around major championships.
Need I mention the most obvious example of a person with minimal dedication succeeding at the highest level: Xavier Carter. The guy ran 19.63 (at the time, the 2nd fastest 200m EVER) skipping practices, playing football (and spring football instead of going to track practice!), etc. He ran 45.x in the 400m indoors 2 weeks after a bowl game. He beat Tyson Gay, Usain Bolt, and numerous others while being called lazy and undedicated by his coaches and teammates and others at his university. In fact, his behavior was so outrageous he is now essentially banned from the LSU campus for causing such problems.
Now, he has been surpassed by athletes who are more “dedicated”, but it shows you how far talent alone will take you. In fact, had it not been for injuries, he may very well be the fastest person in the world right now. Ironically, him getting hurt was when he started to attend more than a few practices a week and eating something other than chicken wings (according to what he has said!).