Keoke,
For once I agree with you, actually, maybe several times. I've seen many a car running around with DIY upholstery & tops. Not to say that it can't be done, but 99% of them look just like what they are. Maybe ok on a low dollar driver, but not on a high quality car.
There are just a lot of inside tricks of the trade, tools, & knowledge gained from hard experience that give a good trim shop a noticeable advantage on the end result.
D