Sure, they'll sell replacement cables, but they make a lot more "free" cables that sell with their phones and tablets. The cost/quality of the cables they package in phones matters to them a lot. They don't care a ton if additional cables sell well or not. And you're absolutely correct that many people will buy Apple official cables just because they're Apple official.

But here's the thing: Apple can't go selling premium cables and including cheapy ones in their phone boxes. The Apple brand has to be viewed as premium all the time, so they put a mediocre cable both in the box and sold on their website.