That would make sense _except_ apple sells their cables on their website. They have already chosen to make selling cables part of their business.
What's more likely is that the quality doesn't matter. Lots of people only care about the official apple brand, so apple can get away with selling an interior product for more money.
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.
What's more likely is that the quality doesn't matter. Lots of people only care about the official apple brand, so apple can get away with selling an interior product for more money.