thanks for sharing Sameer.
I think many students do drop out because better offers come along. 4 years is a long time.
More specifically, here are some reasons I can think of.
1) Some students are interested in research, but do not understand the nuances of what it will entail. Reading journals in the students area of interest is very very important. Also, I think a lot of student underestimate the emphasis that universities place on academic research. You will hear the "Publish or Perish" slogan often in academia. As an academic, you are on the faculty to teach, but high quality research is a primary component of getting tenure.
2) Some unfortunately drop out because they fall short of the academic standards of the institution. This is usually a result of performance on the comprehensive exam, usually conducted after the second year of the program.
3) The rewards are not immediate. Academic research sounds easy. Define the problem, show how your paper contributes to current literature and show that you indeed made those contributions by using models and experiments. However, a typical student fails at the first 2 attempts in writing a paper. A student belongs to one of these three categories.
a) Not good at math, not good at discovering interesting research questions and not good at writing either. This is unlikely, but it does happen. As you can imagine, such students do not stay long in the program.
b) Good at math, not good at discovering interesting research questions, but good at writing. This is probably the average student.
c) Good at math, good at discovering interesting research questions, and good at writing, but the student cannot convert the research question and intended contribution into a model that adequately incorporates the richness of the problem without oversimplifying it. This is probably the standard for a student attempting research for the first time.
If you can get to phase c) by the end of your first year, you are my hero.
4) As Sameer correctly pointed out, it would take a lot of motivation to keep going. 4 years is a long time. Things change. Changes in personal life or professional ambitions can play a very very significant role in such decisions. Most are married and the Ph.D is an acid test for any relationship. For single students, its probably not as difficult, but a supportive spouse can have a dramatic impact as far as keeping your sanity is concerned. It will be a great support system as you navigate through the emotional roller coaster.
5) Others simply find the time commitment too much. It puts a strain on relationships and finances. Most expect to graduate in 4 years. But its no longer enough to graduate. IF you are interested in getting hired to a top school (who isnt?), an acceptance of a journal article at a top journal in the field is fast becoming as important as the Ph.D itself. Very Very exceptional and lucky students can complete this publication "requirement" in 4 years. By exceptional, I mean a student who hits the ground running as far as research is concerned.
If everything goes right, this is what should happen for a student interested in publishing in a top journal.
End of First year - Have a written research proposal ready for the first paper. It is difficult to know what your dissertation topic is going to be right in your first year, but you should make every effort to ensure that this research will form a part of your dissertation. It will help you a lot.
In the Fall/Winter of second year - Complete the paper
In the Spring of second year - present paper at a conference and include it in the conference proceedings. -- A great way to get early feedback on the quality of the paper.
Summer of second year - Incorporate feedback and revise paper. Edit/ Re-edit paper to make it more readable.
In late summer/ early fall - send paper for publication to a top journal.
Early Spring of third year - Accept/ Revise and Resubmit/ Reject decision. Lets make the optimistic case and say "Revise and Resubmit".
Submit revisions by Summer of third year
Late Summer/ Early Fall of 4th year - Decision on paper. (hopefully an accept)
Early Fall - Hopefully, you did not forget about your dissertation that you should have been working on. Defend your dissertation proposal if required by your university BEFORE going on to the job market.
Fall of 4th year- Go on the job market. Preliminary interview with Big Name Schools at conferences. Explain dissertation work.
Spring of 4th year- Hopefully that leads to a campus visit.
Late Spring - Accept job offer.
And all this assumes that you did well on your coursework and passed your comprehensive exams.
You will do yourself a favor by spending as much time as possible preparing yourself before you even begin your Ph.D
Hope this helps.