Frameworks are frameworks, not libraries. You can't just start writing/understanding them - frameworks are different from libraries precisely because they call your code, not the reverse.

Some people hate to write the same basic things over and over. That's where a framework excels. Write only the glue/logic you need to make your app work.

The way you have described your experience with Spring Boot seems to imply you did not take the time to learn it at all, and therefore its' unsurprising to us you had a hard time of it.

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.

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.