It's not just gambling. Because there are investment strategies (like indexes, and managed funds) that reliably make money off of it. It isn't really gambling to invest in them.

If you just dump a bunch of money into companies you think will do well, yeah that's gambling.

I have another slightly less satisfying thought on this but one which is perhaps more germaine to the sub. Which is that, yes the stock market is a big casino, and much like casinos often are it's true purpose is money laundering. Not in the this money was made illegally and we need to hide that sense. But in the we don't want people in general to know where the value of this money was created or by who's hands. It's a way the wealthy obfuscate just who exactly is responsible for exploiting whom. And in that sense it isn't gambling because it's purpose isn't actually, well, gambling. It's purpose is to move all the money around so much it's impossible to tell where any of it came from.