Always has been.

Not always. It has been for longer than we’ve been alive, but stock originated as a way to fund merchant voyages - you paid a share of the costs and got a share of the proceeds (in merchandise or in the sale value of that merchandise) when the ship came in.

Literally the origin of the phrase “your ship has come in”.

Then people started speculating over the future value of and trading those shares while the ship was still at sea, then the concept got generalized beyond merchant voyages, etc and here we are where it’s more like the art market where things are worth whatever someone will pay and that value isn’t necessarily tied to anything concrete.