What is so bad about tooting your own horn or claiming your successes?

Why is it considered low class or poor sportsmanship to tout you winnings or success? As Mohamed Ali said; “It ain’t bragging if you can back it up”. If you are like Shaun White, Simon Ammann, Michael Jordan etc why can’t you claim the title of the best? Why do you have to wait until you are old and not the best that you can say how great you was? When in sports your time at the top if it ever comes is not all that long, you will eventually get replaced by another. While you are still great not to say so would be a waste, after you are old or too old to be the best and try to say how great you was seems a little like trying to hand on to something you no longer have or try to convince people who weren’t there to know you were.