If it's in an applied area, like calculus or statistics, there are usually plenty of books that skip the theory and simply give you recipes to use. If it's a more theoretical area then the theory is presumably why you came, so of course they aren't going to skip it. It's also common to study application-style first, then theory later. So first you take "calculus", then "intro to analysis" which is the same topics as calculus but with the proofs done carefully, and then a more theoretical and abstract approach to analysis.
If you start with an abstract textbook, you'll see a bunch of definitions and axioms at the beginning that might make the book feel self-contained like you can start from nothing, but in practice you are supposed to know the basics of the topic before you start such a book. Otherwise you will have a hard time. That might be the difficulty you've encountered.