This is what I have observed, and this isn't just kit cars, but any car build.
I think some of the issue have already been discussed so I won't recant those.
Sometimes people get started on a project, its new, and they have a lot of energy, and dreams about the day they will drive it.
Then they start to take what they were going to do too far, like powder coating the frame, or changing out the live axle for an independent. Building a bigger, stronger engine than was first conceived.
I call that the snowball effect of which I have been guilty. Then over time, and money the project, along with a wife that don't understand why you always have to modify a $2k part to fit, starts to drag on you. You have to MAKE yourself go out to the extremely hot, or cold garage to work on it.
Then you have the too good to be true deals, that you just have to jump on. A local guy puts a 68 Mustang fastback for sale at a really cheap price so you buy it. Now the money and time that was allocated to the main project is diverted to the new project.
Like I said earlier, the new project is NEW so you have all of that adrenaline for the new car so you put all of your energy into it, putting the earlier project on the back burner.
Then there is the time factor. I have been working on an Eleanor Mustang for over ten years now. Bad part is that its really pretty much done. All I have to do is wire up a few relays for the fuel pumps, and hook the hoses to the F1R supercharger which sits on top of a 540fe tunnelport engine, and I can at least drive it out of the garage. BUT, those last little things seem so mundane that I keep putting them off.
Here is a guy who I've been following for a few years now. He's building a really radical 80's model Mazda truck. But, IMO, he is in the taking it waaaaay to far crowd. I would like to see him finish, but I don't think it ever will be.
Here is his videos, there are over thirty so you can watch them one at a time as they are pretty interesting , and the kid does have some really good fab skills.
Midmounted Mazda Minitruck - Day 1 - YouTube