Casinos increase their house edge in blackjack by altering rules that make the game less favorable for players. For example, they may use more decks, reduce the payout for a blackjack from 3:2 to 6:5, or require the dealer to hit on a soft 17. Other changes like limiting doubling down, restricting splitting, or removing the surrender option can also tilt the odds in the casino's favor. Additionally, using continuous shuffling machines makes card counting ineffective, further increasing the house edge.