Maybe because they didn't think to?

Different people prefer different formats for information. Some people prefer to read the manual from start to finish while others learn better seeing the same concepts in a hands-on tutorial.

From reading the sample of OP's book, it seems far more practical and accessible than the bash manual, so I'm not surprised that a lot of people would read OP's book that have no interest in reading the bash manual cover to cover.

Because generally a manual is “here is every single detail of this thing”, while a book like this is “here’s an overview of the particularly useful stuff.”

I’ll go to the manual if I’m trying to understand how a particular thing works, or how to do a particular thing, but it’s not as useful to me for feature discovery.