My guess, and just a guess, is that it gets down to the time required to create the designs vs the creators individual need for the part and the grey area respecting the designers copyright. I'm just getting started in the 3D printing hobby and until I got my feet wet trying to design my own parts I had no idea the amount of time it took to create something like a radial engine or an airframe. In some cases hundreds of hours are spent. In a sense, it would be like buying a book, making copies of it, and reselling it for a profit. On the small scale nobody really is going to care, but to scale that up to a profitable business would be pretty unethical.
Then you have to take into account that a complex part with multiple pieces may take 30-40 hours of time to print. It's the same problem it has always been is that the Model aviation hobby is not really a moneymaker and people make these parts, pieces and planes for their own enjoyment and that from a business aspect most people are not willing to pay a competitive rate to make it worthwhile from a business standpoint. I have a new respect for guys like
@localfiend here, who design spectacular airframes, spend hundreds of hours perfecting them, and then give the files away for free. It's really very generous of them to do so and shows a commitment to the hobby that goes above and beyond what many are willing to do.
Years ago I was asked by a club member to build some scale scratch-built airframes. I wasn't willing to build planes I wasn't interested in for less than a dollar an hour and he wasn't willing to pay even half of a reasonable rate for my time. He eventually did find someone willing to work for peanuts and the whole thing turned into a finger-pointing session and lost friendships when the plane was crashed on the maiden flight because the owner of the plane installed the radio with the ailerons reversed and crashed the plane.
Everytime I have tried to take a hobby and make a business, it quickly sucked the fun out of it and just became another job.
With that all said, I think if you dig around a bit there are lots of folks with printers who would help you out and there are a lot of "free" files out there that could be printed for you in a piece by piece situation by a hobbyist. I recently printed a part for another member here, not for profit but rather just to help a friend out. For that matter you can get yourself set up with a printer for about the same cost of buying a new transmitter and plane. So the barrier to entry has really come down in that aspect.
