is it not "undermining" them if every exchange launches their own? is there any reason for it? 
It is not. I don't know the sole reason but the competition is extending up to that point, they aren't only into operation but also with the coin that they are producing. Which causes them to think that they have come up with a better coin that has the solution to its current problem.
In fact, they have come with the same idea without any differences. None of them even publish their audit if each of their coins backed by a real dollar. This is why the price of the stable coin can go even lower. Remember when tether and any other stable coin go below $1. Just like $0.9 for example and it doesn't make sense.