Why are seemingly important topics not covered in textbooks?
An important distinction that needs to be considered, in the design of any course, is the timescale over which the knowledge is expected to be applied.


At one end of the spectrum are courses where the knowledge is expected to become obsolete only on a time scale of centuries, such as calculus or Newtonian physics (yes, they change: try reading a calculus text from 100 years ago and the notation is likely to be somewhat confusing), but are generally about fundamental knowledge and core principles and thus often rather removed from immediate applicability.
At the other end of the spectrum are courses where the knowledge is immediately applicable, but where it may become obsolete quite quickly, such as training in particular software systems or laboratory equipment.


Institutions then tend to organize themselves along this spectrum according to their goals:


Strong universities tend to focus much more on the long-term end of the spectrum, because they are trying to give their students skills to last a life-time, and indeed to help those students become the ones who create new systems and applications that render the shorter-term knowledge obsolete.
Vocational schools tend to have a mix that aims to give the students both immediately applicable skills but also enough foundations that they won't need major significant retraining for a number of years.
Professional development and other "on the job" training tends to be much shorter and focuses on the short-term skills that are needed now and where it's OK if you need to train again on the new system in six months.


Now, let's consider your example of search engine optimization: this is a rapidly moving target, since it's based on adapting to the current methods that the current search providers happen to be providing how the internet happens to be organized right now.  Cutting edge SEO techniques are being invented and destroyed constantly.  Pretty much any SEO technique from five years ago is totally useless today, and SEO didn't really even exist as a field ten years ago.

As such, you should expect a marketing textbook or course at a good university to pay little attention to SEO, but instead to focus on more foundational principles.  On the other hand, if you search for SEO textbooks and courses online, you'll find a lot of more vocational or corporate resources available.

In short: educational resources adapt to the timespan over which they are intended to apply.  The longer a timespan an education is intended to cover, the less it will focus on "hot topics," and a good undergraduate degree is intended to last your whole life.