Why do casinos use several decks in blackjack?

Casinos utilize multiple decks in blackjack primarily as a strategic measure to deter card counting.

Card counting is a technique used by skilled players to track the ratio of high cards to low cards remaining in the deck. Because a higher proportion of high cards remaining in the shoe increases the player's statistical advantage over the house, counters adjust their betting amounts based on these fluctuations. By using multiple decks and reshuffling them frequently—well before the entire supply of cards is exhausted—casinos effectively minimize the ability of players to track these changes in card composition, thereby protecting the house edge.1