Gambling is about luck, greed and addiction. Casinos know it well and earn lots of money on gamblers.
This is why no casinos are going bankrupted because they know how the gamblers play. They know that there will still be greedy gamblers and they take advantage of it. For me, it is the most profitable business 
. You can get a huge revenue in the long run but it needs a huge capital to build oneWell, technically it is not quite so
Many casinos bankrupted in the past for being unable to properly manage their bankroll (the forum remembers that). Running a casino in a safe and efficient way is not as simple as it seems. If it is a matter of luck for players, it is also a matter of luck for the casino (minus the house edge, obviously). The point is, one big player can make a bet so high that it will eat away half of the casino bankroll if it wins. That's why casinos set a limit on the max bet amount (read, it is a complex matter), but it is not just that, though
I agree, it's a common misconception that casinos always make money. We can see how many of them failed, reading this forum alone. I have an impression that most newly appeared gambling sites vanish after 2-3 months of operating, with their owners losing thousands of USD. And if they continued to operate, they would lose even more. And in many cases their overall profit is negative not because some user won big unexpectedly, but because they spend much more money on free rolls, faucets, promotions and technical support then their site earns during the same period of time
That's to be expected
Honestly, I wasn't a frequent visitor to the Gambling section of the forum till some time ago (though I remember the times when people were comparing casinos open for outside investments), but there may have been dozens of these that went belly up soon after the launch. The most obvious reason for that is the lack of proper bankroll management which ruined so many casinos out there
But it is definitely not the only cause of them shutting down as you correctly point out. Even if you have sufficient technical expertise in the field (I mean all that incomprehensible stuff with probabilities and what not), this doesn't guarantee success. You should also be able to make your casino attractive to the community so that people like playing there. And it goes well beyond simple knowing how to calculate probabilities (even if it is not simple)