The process is not perfect,If we shoot for UU poke and turn into Nu .It is waste of time and effort.We can not guarantee if a poke will be UU or OU.We shoot to make an Ou poke but the metagame will choose what happens to a poke.
We try make an OU and becomes UU that is fine
We try make an OU and becomes BL it is fine
We try make an OU and becomes Uber it is fine
We try make an UU and becomes NU we wasted weeks maybe months of our time making it and probably waste weeks and months trying to fixing it.We honestly don't know if poke will be OU or not but shooting higher gives it a better chance of having some use.We can't tell what will really happen look at Alakazam on paper it looks like a monster but in reality it is not very good in this metagame.Skarmory has average stats but it is common poke.We shoot for main ou game and if it is stronger or weaker it has a place.
Finally why create a pokemon that will not get used ?We already several pokes barely gets used does make any sense to add more.95% percent of the people say i wish UU poke A had this move so i could use it in OU .Nintendo does fine job of putting pokes that we wish where in Ou but screwed them.We don't need to help them
A counter must be able to both switch into the pokémon it's supposed to counter, with little to no risk to itself, and also pose an immediate threat to said pokémon.