publications - Why don't more academics write textbooks?

For example for an undergrad CS/ECE course, say Intro to Computer Networks, there are about 5 textbooks published by the top 15 publishers in the last 20 years. Why aren't there way more textbooks, since there are probably tens of thousands of people (academics) who have the necessary knowledge to write such a book.

Is it because:

1. 
   

   publishers don't publish a book about a topic for which they already published a book previously;

   
   
   


2. 
   

   or academics don't think writing a textbook is beneficial for their careers and royalties are very low?