You do this 18 times because you have 18 different apps with similar requirements but deal with different data/endpoints/whatever.
On your 19th app you decide to standardize how you cobble together all of these libraries so that you don't have to start at ground zero every single time.
Now you've invented a framework.
How else would you architect with this in mind? Given that literally every other framework is quite similar (RoR, PHP's solutions, etc).
There is another niche, the HTTP server libraries, but they are much more low level.