I may be suffering under the Illusion of Transparency but if you work under this assumption; Most ways of getting rich involve providing people with a good or service that they want more than the money that they pay for it, then it will be true for most cases.

Also, utilitarianism. If you are an outrageously successful monopolist and extract $100 from the billion richest people in the world, what could an effective altruist do with that kind of money? Attempting to make money off of poor people is a bad idea if you can get it off of rich people instead; they have more, and they're freeer with it.