Because there are still gigantic centers in the league that are 7 feet tall without shoes, or very close.
Eventually there's going to come a center that is a legit 7 footer that has a mean game to go along with the size advantage, and in that instance a 6'10" (in shoes) dude is going to be overmatched. Anthony Davis is basically the same height as Towns & Okafor, but the Pelicans realized playing him at the 5 long term isn't the best idea, same with Aldridge who is actually taller than Towns/Okafor.