I've been in the gambling industry (working) for 25 years, and prior to that grew up with a dad who sold gambling machines for a living, so I have plenty of experience. Whilst of course there are variants in the short term, over a period of time, the house will retain it's edge in various obvious, and not so obvious ways.
It's a fascinating subject and one that is a complex mix of mathematics, human emotions, Phycology and much more. Without getting into the vast complexities of how gambling venues/games/sites etc go about exactly retaining an edge, the simplest way to look at it is that the longer you play, the more probability, edge and maths work in the houses favor.
If you toss a coin five times, you may get 5 heads or 5 tails, toss it 100 and you will get more or less 50/50.
that is the power of probability. In the short run, a lot of people manage to win a few times and start to believe that they are able to constantly gain profit from gambling. So they will end up keep visiting the casino to gamble. However, they will all end up losing cause the power of probability will ensure that they are unable to profit in the long run.
Agreed with you, someone has to be very lucky to won big money but its not possible all time. One can win huge amount but later on if he continues with the gambling then in the long run he will loose more money than what he has won in the past.