I'm not a builder, but I have owned a couple of planes. Two things come to mind about building before learning to fly.

First is, how do you know what you want? When I was learning I thought a C150 would suit me fine. After enough hours of going slow, I wanted a Grumman. After taking people up with me and needing to carry a little luggage, I wanted an Arrow. After seeing the maintenance requirements I wanted a Warrior. After trying a few small airports with rough grass strips I wanted a C172. What I ended up with was a TriPacer. Funny thing is I never thought about them when I started looking, and I can't think of anything else I'd rather have now. All the other planes stopped me from doing at least one thing I later found I liked doing. Deciding what your mission is when you have no experience seems a little odd. Same for spending years building a high performance traveling machine and finding out you like slow cruising to little airstrips for pancake breakfasts.

Second is, how do you know what the cockpit should be like or what "improvements" you'd like to make? If I had a nickel for every RV builder that stuffed their panels with enough gizmos to make a 747 driver jealous, only to find out they just use the gps, I'd be rich. How do you know what the sight picture for landing is like before you decide to build the seat with a few extra degrees of tilt? Is a constant speed prop worth it to you? Really? Or are you just going by what other people said? Do you want to be one of the cool kids with a taildragger or a girlieman with a tricycle that has never had to worry about a groundloop? As a builder you are free to make changes, but it seems like a waste to spend months getting something perfect only to find out a different configuration is needed.

For me, getting a different airplane just costs money. For builders, I'd imagine there would be a little pride lost in realizing you built the wrong plane. Maybe to the point of suffering through it's drawbacks instead of selling or building another?