You don't have to trade against the firms, retail can make up the majority of some markets (e.g not on dark pools etc..), you just have to beat them and join the firms in taking their money. It's far more difficult than buying an index but it is not a scam just because it is hard and most people don't know what they're doing.

Edit to clarify: my portfolio is almost all index funds to clarify, and that is what you should do too, but I'm just saying it is entirely possible to trade and it is a skill. Something can be somewhat like gambling and yet still have elements of skill just like poker.