Because it's already been explained.
Mooy plays considerably further up the pitch than Stephens.
He spends more time in the opposition's half than his own.
That doesn't sound like a direct replacement for Stephens to me.
Missing point here perhaps is that our formation is different - even against Brum and Valencia, Stephens was higher up the pitch than we're used to seeing in the bus-parked formation of last season. The question is which of our 4 CMs will be most effective under Potterball, recognising tactics will be fluid. Could be a horses for courses question as we set up differently against differing opposition, or even during games. There is no 'straight swap' IMO. Mooy wouldn't be a 'direct replacement' for Stephens if we were deploying the same tactics as least season, but we're not ?