Indeed, copyleft is not enough to guarantee that corporations won’t make money with your code. Without modifying the code, an evil company could increase its profit just by using your software.

Correct, but depending on the specific license, they would still have to distribute the source (read: not JUST credit you, THEY have to independently provide a way of getting access to the source code), and if they make any improvements, which they’ll probably want to do, they have to share that back with you. This results in the product itself continuously improving for everyone, profit or not.

If there wasn’t a license term against it, companies will be greatly incentivized to keep any improvements they made closed and therefore their product will be better than the original open source product, which will give their now proprietary product an edge in the market, which has historically shown to be enough to snuff out the original open source efforts.