Because a PhD represents a level of education in a subject notably beyond a 2 or 4 year degree. You can only absorb information so fast. I mean yes, if you complete all of the degree requirements before six years and the college allows you to you could complete it faster, however again, there’s only so many hours in the day and even a six year doctorate is going to eat up your life pretty well.
Depends on the field and the program. I did a bio PhD so I can comment on that. Our rule was essentially two first author papers or equivalent. That takes quite a bit of work, and it does work out to 6 years on average.
I took 5 years. I knew at least one person who did it in 3. Some took 9, 10, or 11 years (although many programs stop funding people after a certain number of years).
It also depends on the major professor. Some will simply not let you graduate “early” in their mind.
It works differently in different countries. In the UK doing a PhD can take as few as 2 years though normally takes 3-4 and pretty much the only requirement is to write and defend your thesis.
Generally you shouldn’t do a PhD if you want to improve your job prospects, you do a PhD because you are truely interested in a subject and want to further learn about and eventually expand our knowledge of a specific area in the subject.
For physics the PhD program usually consist of 2 years of post graduate level courses on modern topics. They are covering similar things as your undergrad courses, just fill integrating calculus and differential equations as well as showing you less of the “classic” examples.
Then you take your qualifying exam to verify you really know that material.
After that you work under a faculty PhD on developing an independent thesis topic. This can take a year or two depending on the student and the field. Then it takes a year or two to do the experiment and write the thesis.
And a thesis is a different best than most research papers. It must explain *everything* involved in the experiment. It can’t leave our details other papers do that are “standard practice” or settled concepts. So instead of being a few pages long of just the important findings and methods, it’s a 100-200 page book.
The student must then defend thier ideas as faculty try to tear it apart or just plain stump the student. So you have to know it very well.
Clarification: average time to degree in non-lab fields is *even longer*, because you have to do fieldwork or archival research to collect the information for your dissertation (which, in every academic specialty, requires original contribution to the shared body of knowledge). Also, US degrees often take longer because of the reliance on graduate student labor for teaching (not just research assistance).
Some countries have doctoral programs “by research” that assume you’ve done the relevant coursework prior to tackling the dissertation project. That’s not really a shortcut so much as it is a higher bar to entry into the PhD program (you have to do the same prep, and it takes the same time)
Mine (biopsychsociology) took 9 years. A lot depends on where you are. When I was defending my Master’s thesis we happened to have a visiting professor from Slovakia who wanted to see it. When I finished he told me that the same level of research and time would have earned me a PhD at his university.