They make that as a commission for separating end users from their money, and Apple has made that a really seamless process.

But while developer support certainly is correlated with that income, it's not tied to it closely enough to make it a priority. If they were to spend an extra $20M/year on developer support, would it really lead to the extra $67M/year in app purchases needed to just break even? Whereas if they just directly charged for premium developer support, they could set the price to whatever it needed to be for that business unit to make a profit.