If you play blackjack, you've probably heard that fewer decks are better for the player. And if all other rules are equal, that's true. The house edge is lowest on single-deck games, and increases with each deck added.
To make it harder to keep track of the cards in the deck, casinos soon switched to a multideck game, with two decks shuffled together. Today, most casinos use six, eight, or even more decks shuffled together. This is why few dealers deal by hand.