1. It loses its value because what paltry profit it offers has to be made in the first sale due to resales. How many professors make money off their textbooks? Very, very few. For most, by the time you compare any profit to the time they put into it, it was a waste of time. Typically only makes sense as a labor of love.

2. It's the only way to make it worth the authors' time.

3. Many are consultants, etc. And perhaps some find the subject interesting and enjoy teaching or research or enjoy the lifestyle. Your logic could be applied to most fields. Thankfully, it's not just incompetent practitioners that choose to teach.

4. How many academics are making money? Universities are not paying extravagant salaries; they're moving toward paying adjunct faculty pathetic amounts of money to teach an increasing portion of coursework. You get paid less to teach a subject or practice in a university setting than you would in a non-academic setting.