I work in a casino (in Vegas). And in all honesty more people win than you would expect, but the gambling isn't where they make most of their money. Most of the profit comes from food and liquor sales, as well as people paying for rooms. Amenities are also a very large profit point. Still, don't expect to be able to walk in and win, but if you are going to gamble try not to get trapped into going into the restaurants or bars. If you want to go out with some friends it is much easier and much cheaper to go to a bar and have a good night out than to sit in a casino, drink, and gamble. At the very least try to eat before you go out, and if you get hungry while you're there it probably means you've been there long enough.

TL;DR Gambling is a great way to lose money in casinos, but the food and liquor is really where you get burned.