That's just how math is done in research. If you don't like that then you'll hate, even more, pure math papers where they start with a theorem then show steps to end up with a true statement that is the theorem.

The intuition behind how the author came up with that path of thinking to come up with the final theorem is left (justifiably) entirely in the authors scratch paper or notebooks.

Some authors do give out insight onto the steps they took or their general intuition which is always nice, but not a requirement.

It's also worth mentioning that a lot of us like doing research but don't like writing research papers (as that is just a necessity due to humans lacking telepathic communication) so giving out more info is an optional step in a disliked process which makes sense why it's skipped.