By competition, you mean to spend double the price per round that a 5.56 gun will cost, plus deal with the significant bullet drop past 300 meters? There are those who've tried it with 7.62 x 39, and on the longer stages they give up points because of it unless they have shot tens of thousands of rounds.
Of course, if you reload, all that means much less. Still, very few go to matches using cartridges that drop more and where they can't pick up their brass. They take advantage of every little detail because that is what separates first and second place.