“Aren’t all books text books?”

Students read from textbooks, but why call them “text” books? Don’t all books—except picture books and blank books—have text?
 
“All this news about Texas text books got me thinking, aren’t all books text books? Isn’t that what books are about? Text?” was posted on Twitter on March 18, 2010. “Why are school books called textbooks? Aren’t all books text books?” was posted on Twitter on April 4, 2010.
   
             
Wikipedia: Textbook
A textbook or coursebook (UK English) is a manual of instruction in any branch of study. Textbooks are produced according to the demands of educational institutions. Schoolbooks are textbooks and other books used in schools. Although most textbooks aren’t only published in printed format, many are now available as online electronic books.
     
Twitter
T_Gibby‏
@T_Gibby
All this news about Texas text books got me thinking, aren’t all books text books? Isn’t that what books are about? Text?
8:47 PM - 18 Mar 2010
   
Twitter
the brein‏
@the_brein
Why are school books called textbooks? Aren’t all books text books?
4:43 PM - 4 Apr 2010
 
Twitter
VerbalHaze‏
@VerbalHazeNugs
Aren’t all books textbooks?
2:30 PM - 18 Apr 2010
   
Twitter
John Peary Coppin‏
@pearthebear
Isn’t it odd that “textbooks” are called such? Aren’t all books textbooks?
12:58 AM - 25 Aug 2010
 
Twitter
Raymond Angelo‏
@howtoraymond
Technically, all books are text books. Because they contain text.
6:21 AM - 23 Apr 2012
 
Twitter
Joseph Castagnino‏
@Eselhut
Why are they called textbooks if all books are text
4:53 PM - 12 Jun 2012
 
Twitter
chino liao‏
@chinosupersized
If all books are text, then are all books textbooks? #thinkaboutit
11:48 PM - 21 Oct 2012
 
Twitter
Victor Anastacio‏
@VictorAnastacio
All books are textbooks. Except picture and blank books.
3:30 PM - 5 Dec 2012
 
Twitter
Comedy Cartel‏
@comedycartel
Isn’t “textbook” redundant? All books are textbooks. Except picture and blank books.
11:30 PM - 5 Dec 2012
   
Twitter
Victor Martinez‏
@Jedi_vic
#danfact #1: All books are “text” books
1:00 AM - 8 Mar 2013
 
Reddit—Shower Thoughts
Since all books have text in them, aren’t all books textbooks?
submitted

Title kind of sums it up.
   
Reddit—Shower Thoughts
Technically, aren’t all books “textbooks”?
submitted January 17, 2016 by DutchK1d
 
Twitter
Maddie Rundlett‏
@madsruns
That feeling when you’ve been in class too long and you start thinking things like “damn…if u think about it. All books are Text Books”
11:36 AM - 18 Jan 2017
 
Reddit—Shower Thoughts
Aren’t all books text books?
submitted October 5, 2017 by kuzzi09