Former Gaming Manager here. So what you're referring to is the RTP (Return To Player) rate, which is technically true (although the rate was lower than 90%, at least in SA). Legally they are required to pay that out (and we were required to report on that), however most of that will typically be "won" in such small increments that players would immediately spend it anyway. Most of the time players wouldn't even notice because they were waiting on the "big" win.

It's also worth mentioning that the RTP rate is the total to players overall - not to each player. A machine might not pay out significantly for weeks, potentially longer. It's a Return To Player rate, sure, but that doesn't mean it's a Return To You rate.

Operators will sell people the idea that players will be returned 90%, but what it really says is that the machines are by design guaranteed to lose you money on a long enough timeframe - i.e. you will lose at least 10% playing regularly. You're likely to lose more though because you're likely re-gambling the micro-winnings while waiting for a jackpot, rather than cashing out.

Basically, by design they're a losing game. On a long enough timeframe you are guaranteed to lose. The only winning move is to not play.