They can and frequently such people are; but they aren't very obvious because they stop while the business is still small. If we don't harness greed into doing something productive then there isn't enough motivation in the world to power a modern industrial society. People like Bezos would just sit around running a local bakery or something instead.

The way the system works is people who create unfathomable amounts of wealth get to keep unreasonable amounts of it. If that link is broken, they'll stop at creating a reasonable amount of wealth and then everything grinds to a halt. If someone is in a situation where they are doing a good thing they should have every incentive to keep going and not stop.