That's because they have more money. You have to pay for the necessities, and then you can save. If you're making near minimum wage, you won't be able to save much.
Also, those investments, like stocks and rental properties, transfer wealth from the poor to the rich, generally.
Sure, no one is disputing the fact that rich people have more money than poor people. For many reasons the rich get richer at the expense of the poor, the OP was noting one of the reasons.
Another is that the tax code incentivizes making your income through investments, something unattainable for people who aren't rich.
Also, those investments, like stocks and rental properties, transfer wealth from the poor to the rich, generally.