Why don't casinos just use a different deck every time to eliminate card counting?

They have found if they use three decks and shuffle every fifth hand no one can win by card counting.    The idea is to have as many hands in an hour as possible as that increases the profit.   Having to stop and switch decks will slow things down. 

Casinos are competing with each other for players.  That's why they offer free drinks, have inexpensive dinner buffets, and will give away lavish comps to players who win (or lose!) big.  They want you in their place gambling away your hard-earned money, and not in the place down the street.  With that in mind, why would I want to play blackjack at your casino where a freshly shuffled deck is used for each hand when your competitor next door is advertising single deck blackjack with no shuffles until the end of the deck is reached?

Because here's the thing.  Most people who count cards still lose money.  Either their system is flawed or they can't cope with the noise and confusion and distractions in the casino.   So they lose, but because they mistakenly believe that the odds are in their favor they'll stay at the table and keep losing long after other people would have given up.

They have found that a six-deck shoe will give them greater variability than shuffling a single deck.

BTW:  if you bet the min the all of a sudden make a big bet, don't be surprised if the pit boss comes over and orders a reshuffle.  That's why they used teams and signal s in the movie "21".

They look forward to all the people who think they've found rain man and blue a fortune realizing they were wrong.