Burnout is definitaly a factor, you're right about that. Personally, I had quite a bit of success (serendipity) during the PhD years and so I was having a lot of fun and I didn't wanna leave the lab. At the same time, when I was "writing up" for 1-2 months (which was not an ordeal by any means because it involved cutting and pasting my publications as chapters so in essence, writing up only took 1-2 days), I spent a lot of time playing video games, sitting on my ass, going out to the bar, traveling (to different bars), etc. Those days were fun and definitely something I needed. I think that if projects don't work out the way they're supposed to do and you're spending years and years in the lab being not as productive as you would like, then burnout becomes a significant deal. Sure, your love for doing science and troubleshooting can get you through some of those 🙁 days but the human will can only take so many punches and shots. I have seen burnout in some of my fellow MSTPers at my institution, mainly because experiments haven't worked in a long long time. They've been in the PhD phase for too long that they get the sense that it's too late to quit. However, they also realize that if they stay the course, they still may not see the light at the end of the tunnel for quite some time. It's frustrating.
I was cognizant of these issues, but still quite naive, non-cynical, and optimistic when I first started the program. One of the first pieces of advice I got was, "Hope for the best, fear for the worst." This was back in the age when the average # of years needed to complete the PhD phase at our school was around 5 years. When I first started, there was this term used to describe people who were spending 10+ total in the program, "Club X" or "Decade Club". Some folks throw around the term, "Lifer". They have since graduated. The majority of the folks here graduate in 4 years. Some of us have completed the PhD in 3. Some of this is that the program director has been very proactive in trying to get people finished in a reasonable amount of time.