The games are setup to take a percentage. It varies with the game. Slots are some of the worst, IIRC the house take is 10%. Pai Gow poker (depending on house commission and exact rules) is 1%-3%.
If you make bets commiserate with the size of your gambling budget then your expected loss over the long term for slots is 10%. Your expected loss for Pai Gow is 1%-3%.
If you make larger bets than you should given your bankroll or play bad strategies you can lose a lot more. If you get lucky and quit immediately after a big win you can come out ahead.
The house doesn't give a fig if the average person loses or wins; they just need people to play. They are well aware a significant number of people will walk away winners and that makes them happy: it's great marketing!
 (pdf warning) http://gaming.unlv.edu/reports/nv_slot_hold.pdf
Yes pai gow or 21 or (the best odds not including card-counting, IIRC), baccarat have better odds. But, if you go to a premier casino on the strip, you might not find a table you can afford. It's not uncommon for them to have a couple $5 tables and the rest $25 (per bet) and up. Mathematically it's a better choice, but a $25 buy-in costs $500 which you can lose in minutes if you're unlucky. Not a lot of people can stomach that (nor should they, as my original argument was for the entertainment value of gaming).