to piggyback on the post above, world tour races are moving faster than your local race which means the extra .5mph at the pro level requires an exponentially higher amount of power than the .5mph at the local race. Remember that the guys they're trying to beat are also pros, so you need a bigger delta in ability against the top competition in the world.
The fact of it is nobody is strong enough to just inflict their will on a pro race. It takes huge ability AND some sort of circumstance where they somehow allow it to happen.