It’s a simple problem of economics and return on investment, as well as Adam Smith’s famous directive that “nations” (let’s say “companies” in this case) should emphasize their own comparative advantage.
Apple owns the iPhone. Period. No one else can make an iPhone, so Apple’s ‘comparative advantage’ is in… developing, improving, manufacturing and marketing the iPhone. When they invest to grow their capacity, they invest to “improve and make more iPhones”. That’s for starters.
To follow up on that, the iPhone (since only Apple can make it—or will make it, because they won’t license it to anyone else) is a higher-margin product than any case that could be purchased as an accessory for it.
There are a lot of case manufacturers. It’s a wide-open field, with a lot of competition. Even if Apple were to make “the absolute best” case for the iPhone, it probably wouldn’t be a big money-maker (it certainly wouldn’t have the margins or even the same sales volume as the iPhone), because not everyone needs “the best possible case”. People shop for cases (if they even have one) on whatever criteria matter to them: price, style, durability, or some combination of those factors and others.
Why would Apple want to dedicate any of its resources to making a low-cost, low-margin, low-volume seller in a wide-open market… when they could put those resources to better returns making more iPhones, or developing the next big thing for them?
They’d be fools to do as you suggest; and their investors (including myself) would howl with protest and dump the stock, looking for better management elsewhere.