There are many different qualities of textbooks, after all! :)
Many are exactly imitations of older (not necessarily good, but successful) books, perhaps with better graphics, or some other superficial changes.
Some have resonated with for-profit publishers' ideologies, and are heavily promoted.
Even among those which aim to serve their subject, there is often a tendency to be toooo encyclopedic, which has some virtues (for reference purposes), but does certainly heavily mask story lines.
And, yes, sometimes there are monographs ... which can be exactly what you need, if they hit your interests, or can be completely orthogonal to your goals.
It is highly non-trivial to gauge the quality of a book without looking through it in some detail (not to mention having an idea about the relative competence of the author). So, for myself, I've bought (out of my own pocket, supposedly from my clothing budget... :) thousands of books, most of which are high-end textbooks. I've looked through every one, searching for potentially amazing ideas that were previously unknown to me...
EDIT: Likewise, I certainly do also look through a great many on-line preprints for the same reasons...
Yes, some expense, and one of my activities is to try to replace some ridiculously expensive textbooks by my own lecture notes, in several subjects.
But/and both for my own purposes and for purposes of competent exposition, I do need to know whether I'm missing something... especially in fields where I supposedly am expert. :)
True, not everyone can skim through books or large papers quickly. So a strategy that requires that may be infeasible for some. I'd hesitate to "excuse" it, though, as though not doing so were essentially irrelevant.
Yes, I do essentially require my own research students to read most of my own notes, as opposed to explicitly requiring reading of "official textbooks", but this is less forgiving than it might sound, since my own notes do cover quite a bit. And, my people seem to be inclined to look at the standard textbooks in any case.
(This is in math, at an R1 state school in the U.S.)