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.
To follow on this, the way casinos work is to make you think you are flipping a fair coin when you are actually flipping a loaded coin. It's not a coin that always lands on heads. But one that is designed to land on heads 54 times out of 100 instead of 50 will make the person betting on heads (the casino) a lot of money over time.
Also note, that how it's possible for gambling to be profitable in the long term, build a casino and put the odds in your favor.