There is some speculation involved with stocks, but they are fundamentally different from gold, btc, or art because they are productive assets. They aren't just a thing that sits on the ground and does nothing. They represent some small portion of actual factual work done by human beings to produce goods or services. This is why buying the diversified stock market is investing and buying the diversified crypto market is speculating.

There is risk involved in both, but they aren't identical.