It's not about the physicality of a book, it's that guides are typically of lower quality and less in-depth than a book is.
Rarely, you get a "guide" so good it's a book (for JavaScript: Mozilla Developer Network). For many languages and tools, something like this does not exist.
What C++ guide online is going to teach me as well as The C++ Programming Language, 4th Edition? What Rust guide online is going to teach me as well as The Rust Programming Language?
I have read so many tutorials that were just bad, that reinvented the wheel, mixed patterns in MVC, had business rules in javascript without server side validation and so on. Books usually will touch on why something is done a certain way and you can trust an author that is well regarded in the field. I guess it may take a bit more effort.
The Pragmatic Programmer
Clean Code
The Clean Coder
Domain-Driven Design
Growing Object-Oriented Software, Guided by Tests
Continuous Delivery
These aren't "programming books" for the most part.There is a huge difference between "programming" and "software engineering".
And very few people do algorithms and math on their jobs.
You also don't learn most of what the books teach you on your job.
Knowing how to do algorithms and math is rarely what's needed to deliver software.
We've got too many people employed as software engineers when all they know is computer science. They cause a lot of problems without even knowing it. When others complain, they respond that their algorithms are perfect - as if that were where the problem was.
Timeline of features is usually lost when reading about them online.
A book can present better timeline and overview of what all things are available in a framework.
There are experienced people writing books and people are even paying for them. Surely, they do have smth worth mentioning in a book.