Anybody can legally buy stock.

It is substantially more difficult to acquire a large amount of stock if you don't earn a lot of money, since there is naturally far less leftover after your expenses are paid.

And further, stock can be purchased with dividend payments. So the rich can afford an ever increasing piece of the pie even if they don't labor at all.

It's amazing how much you can accumulate by steady investment, and reinvesting what you have.