Its likely a combination of things several have mentioned.. experience, knowledge of the parts and components, disposable income, personality and so forth. Its really in some ways not much different than giving one a choice of 10,000.00 or 2 cents a day compounded daily...
At the end of the day, the airframe is pretty much the cheapest outlay, at least its been for me in my experience. I prefer to fly quality, proven gear. I don't fly a particular servo because I think company ABC is the best or such, but typically because others have had a high rate of success with that brand. Doesn't mean it won't fail, but means that the odds of it failing are higher. Same with all the rest of the parts. I prefer DA, have no problems with DLE, but there are a few I'd never buy, though they might be cheaper. Not worth it to me to try and save 10.00, then spend 50.00 of time re-inventing the wheel getting the damn thing-a-majig to work right. Some items may be made at the same factory, but there can also be different materials and quality controls applied as well.
Guess you could say you have to spend 700 to not bang up a 500 frame.. ha!!!! ..there's a bit of truth to that. Am not going to spend 1500.00 on an airframe then stick a 30.00 orange receiver in and think I'm saving a buck. That's penny-wise and pound foolish.. Maybe in smaller foamies and such I'm not so discerning, but will still stick with the brands I prefer. But most of what I build/assemble are 1/5th -1/3rd, so its not unreal to have 2K or more wrapped up in one, and I'm not going to drill it in because some cheap battery failed or such. Would I ever get my money back out of it? HELL NO... but if you're in this hobby as some kind of investment, good luck. These aren't firearms, but more like new cars.. drive it off the lot and you just lost 10 grand.. Have made some stupid decisions regarding RC purchases.. think most of anyone involved for a few years may have as well at one time or another, so in a way, yes.. you can buy experience! Ha!!!! ... in the end though.. cheap does not necessarily equal good.