Games of chance vs games of skill
In most online casino games you play against the house. The house has a fixed mathematical edge built into the rules, so the more hands you play, the closer your result gets to that edge. American roulette carries a 5.26% house edge, European roulette 2.7%, baccarat on Banker about 1.06%, and blackjack with correct basic strategy as low as 0.5%.
In poker things are completely different: you play against other players, and the house only takes a commission (rake). There is no built-in edge against you — only the skill gap between you and your opponents. That's why poker is the only casino game category where you can be profitable long-term.