Reliability requires a brand presence. Requires trust.
I hear "Ford" and I think "Well, maybe some of their vehicles suck pretty hard, but they've been around HOW many decades? They must plan to be around for a while longer, so I can trust their next car to not suck so hard that they know they will go out of business because of it".
Some guy off the internet who modifies a design? I know feck-all about him and his history. I'm not getting in that car and I don't want it on the road with me. I want LOTS of oversight, lots of regulation, lots of requirements, and that takes manpower and resources and money to test.
So let's say one guy has been in the free-car scene for a couple years, and he's pretty good. Mind you, two years is a SHORT time to become an industry expert, but let's just pretend. Okay, he's working, he's trusted, and...then his savings runs out and he says "oh crap I gotta eat something". Does he take a job where he's getting paid, abandoning free cars, or does he try to get paid by the community somehow? Maybe he says "Well, I'd like five thousand dollars to personally certify a car design". Where does that money come from? Some random donor who likes his work? Riiight. Or maybe he says "I'll help clean up and certify this design, but I want $200 for every copy of this car that gets made". And suddenly they're not free anymore.
Donations isn't that viable either, if your audience is small and not very wealthy. Suddenly you move into an old-fashioned patronage kind of system, where the artists all flock to the rich people and try to become their pets or something.
I want to like the barter system, I really do, but what happens when the peanut farmer wants a new tractor but half the tractor-design team is allergic to peanuts? And how the bloody hell do you handle micropayments? Barter works for big costly things where social capital is still in place, where one guy can get shorted on a deal and everyone knows the other guy still owes him something. If you make jewelry, how do you buy a copy of one song from an artist? I mean maybe you can copy your jewelry too, so it becomes a fair trade, but...maybe you draw portraits, which aren't easy to give to multiple people at once, and take many hours to design...