Hasn't the practical, economical car already been invented?
Sure, but there is also the fun and satisfaction of building your own car and having something that's unique. Also, for some people it can be tough to justify spending $5-25k and a few years of weekends building something that's "just a toy" and can only be taken out on nice sunny days.
My dream car right now is the Smart Roadster
, except legal to drive in the US and without the reliability problems. I spend a lot of my spare time imagining a two seat, mid-engined sports car that gets 45 mpg, rides through hairpin turns like a slot car, and has MGB simplicity combined with Toyota reliability. The problem is there's not a lot out there that comes anywhere close to my unreasonably demanding list, so I sit around and doodle different body styles and different chassis frames and say "I'll bet I could build this and make it work" or "What if I Kimini-fied a Fiat 850 spyder?"
I don't know what Slappynuts' design parameters are. Maybe he should just go and buy a Civic, but as long he can be honest with himself, somewhat realistic with his goals and plans, and as long as he doesn't get innocent bystanders killed, I say more power to him.