However, for this to be true in practice, you would need all customers to be playing perfectly, have perfect money management, and have a combined bank roll equal to that of the casino. Obviously, this is not the case. The majority of people are bad at money management, especially when it comes to gambling, and so the majority of their customers will be poor at managing their bankroll and their bets. Players frequently tilt, bet overly aggressive, think they are on a winning streak, bet bigger and bigger trying to regain losses, etc., all of which works in the house's favor. Their bankroll also isn't combined. The average person who walks in with $100 isn't going to say "I'll walk away when I hit $200". The majority of people would keep going and aim for higher profits. In reality, this means a swing of several hundred dollars in their favor which the casino can take in stride, whereas a swing of only $100 in the other direction is enough to bankrupt them.
Also, if you've ever been in Vegas, you'll notice that when you're playing, the waitresses will bring you quite a lot of "on the house" drinks.
Those drinks pay for themselves. Drunk gamblers are more likely to bet big, play suboptimally, and make mistakes. 
