It is my understanding that most professors who author textbooks do so for some combination of fame or altruism. Perhaps the author of a popular calculus book

It is my understanding that most professors who author textbooks do so for some combination of fame or altruism. Perhaps the author of a popular calculus book (Stewart) might earn enough to justify his time, but professors writing niche texts for upper-level courses likely earn minimum wage at best for their efforts. Thirty years ago, one had to work with a publisher because the technologies required to create bound dead tree versions of your words were expensive and labor intensive. Now, the marginal effort (beyond actually writing the book) to make something look nice is fairly low. Wouldn't both fame and altruism be maximized by producing a free, widely-available text? Edit: Compared to research, writing text books doesn't help much with obtaining tenure, does it?