For most gamblers, that is a good thing. Sure, if you play for a long time your winnings average out to a 5% loss; that's how the casino makes its money. But over the short term the random element means that the slot machine can pay out much more than 100%. So people play in the hopes that they'll get lucky and hit a high-pay-out short-term streak.

No one would play if it paid out 95% on every pull.

Of course, the real gamblers play poker or blackjack, where the odds really can be in your favor, if you develop the necessary skills.