There are many different factors in poker other than luck, from the honesty of the dealer to the player's ability to win even with bad cards. That's why I never considered poker a game similar to slot machines, it's a completely different game.
Bad cards and they will flip and skip the game. It can still work out but that is what I usually see is happening. Either way poker is a different type of game that needs some skills to be able to win by bluff. Slots have you in a place where you nothing to think about or do, just press the button and see your money go down the drain.
Hence some people practice poker and improve their skills in it, but that will never happen in slot games.
I'm almost sure that there are different casinos, some want to play honestly, others will resort to fraud, probably slot machines can also be set up in such a way that winning occurs very rarely...
Fraud casinos would not go too far, they will eventually be caught and owners jailed for fraud. Online, there is very little chance of fraud because games have to be provably fair.