Card shuffling does take time, and a lot of houses use automatic shuffling units to speed that process up. But those units aren't perfect either and will jam and break down.

You also have the problem of players possibly marking cards (bends, folds, tiny nicks, spots) for later use. In the casino's eye, it's a lot cheaper to use new preshuffled decks in every shoe and drop the expensive ShuffleMaster machine.