Why use only oversimplified/ideal equivalent circuits?
The real world is infinitely complex, so all descriptions are approximations. There is no "final and definitive truth", or if it exists we don't know it yet. (A colloquial term is "it is turtles all the way down": you never get to the root cause.)

In engineering we always use an abstraction (= simplified description) of the components we use. That abstraction should be valid within the limits of the problem we are solving. What people (and teachers, and textbooks, and worse: engineers) sometimes forget is to state or be aware of these limits. (This accounts for a fair amount of funny questions we get here at S.E.!)

I don't know your textbook, I can only hope that


the level of description it gives is appropriate for the example problems it shows and for the problems the students have to solve 
it states the limits of the description (probably mainly frequency, but also extreme current, voltage, temperature, etc.)


To finally answer your question: The type of problems that the students are expected to solve is the rationale for the level of description.