I can't disagree with any of the comments made so far... Money, time, talent, life's priorities all can slow a build down to where it stalls. Funny, just yesterday at a car show someone asked me how many of these kits get purchased and never finished. I have to say I don't have the answer, but I suspect it's more than we might think. When people asked me how long my Mk3 took to build, and I tell them two years, many aren't surprised but others say they wouldn't have the patience to stick with it that long. They are not good builder candidates... I guess the only thing I would add to the comments is that I think some underestimate the effort required. Perhaps FFR should take a little blame for this, because I think they understate the amount of time required. I don't recall the exact number, but it's several hundred hours. I guess it's possible, but certainly not for the inexperienced or non-professional builders. This could be a factor for some.
While this is a kit, each is still a custom build, and there will be bumps and surprises along the way. We regularly see forum posts about people "disappointed" because something didn't fit exactly as expected, they had to "adjust" something (bend, clearance, cut, whatever) and seem surprised that this is necessary. This could be a factor for others. Maybe they were expecting it to go together like the plastic models they built as a kid.
For me personally, I built my Mk3 as someone with some mechanical aptitude, but zero experience doing anything similar. I got a bit of a headstart because the car was started by someone else. The two years I spent had ups and downs, and sometimes I just had to walk away for a bit. But stuck with it and eventually finished. I decided I liked the building enough to try another, so now working on a Mk4. Again, purchased from someone else but much closer to how it's delivered from the factory. I'm finding that my experience with the first one has paid off, and moving a little more quickly and fewer mistakes. (really) But it's still something you have to be dedicated to. I commented the other day to my wife that these cars really are a lot of work. Exactly what some of us enjoy and get a giant sense of accomplishment from. But not for everyone.