I've done both and I'll go the other way here. There is a lot of thought, and trial and error that goes into marketing a complete bike. They've taken care of a lot of little details that can be very frustrating to figure out on your own. I did a lot of riding and tweaking, carrying tools with me, making adjustments. For quite a while, putting my own together.
Verus getting home, jumping on and going and having FUN. On a solid, engineered to the job, de-bugged, bike.
The Market is very quickly lopping the fat off the top of the market. (falling prices) and the gap from DIY to pre-built is closing fast if comparing apples to apples.
The more epsensive bikes that most site as egregious example of over priced do in fact use better and more expensive components.
Bafangs are cheap, but they aren't the best quality.