Your utility function need not be completely linear, just locally linear. If your utility function measures against the total good done in the world, your effect on the world will be small enough to be locally linear

Most people don't want to optimize the total good done, but instead care about the amount of good they do. People donate to charity until the marginal utility they derive from purchasing moral satisfaction falls below the marginal utility they derive from purchasing other things. In this case, diversification makes sense, because utility you assign to good you're responsible for is very non-linear.

If you're giving to charity at all, that's awesome. Do what motivates you.