At best, the PhD thesis is read by:
- The author
- The examiners
- The supervisor
- The author's parents
- The author's roommate / spouse / fellow students in research group
Or less than 10 people in total. What, then, is the point of writing it? Writing a complete PhD thesis is a time-consuming process, and that time could easily have gone into taking more courses, doing more research, supervising more students, and so on. Of course the PhD student has no choice but to write one, because the programme typically requires it. However that still begs the question as to why the programme requires it in the first place. if the thesis is so useless that so few people read it, what's the point of demanding a thesis as a prerequisite for graduation?
Somewhat related: What is the point of a PhD thesis whose content already exists in published papers? Still, even if there are more readers because a thesis provides a gentle introduction to the field, it seems more sensible to me to just write a monograph and leave out the thesis.