Actually, I think the main issue would be price. Flite Test got me into the hobby in large part because I can scratchbuild their airframes for next to nothing. That meant a lot when I was starting out, and was reluctant to spend much on something that I was going to crash a lot. I'd guess that if FT provided pre-built models, the added labor would put prices up there with other ready-to-fly models on the market, and their advantage would go away. And if I saw two models at a similar price... one a properly engineered EPO/EPP/balsa type, and the other folded up out of foamboard... I'd go with the former, even if their performance was comparable. Flite Test's niche is to provide plans, kits, and supplies, and they do it very well.
Where are you located, by the way? Maybe someone on the forum will be local to you and can build you one.
Cheers,
Gryf