I built my own systems before, too, and no, it isn't hard. But it takes time, and not everyone has that time. Also, equating intellect with the ability to build a computer is pretty dumb. I can guarantee you that there are very smart people out there who couldn't build a system that makes sense to save their lives.
1. It takes about two hours to assemble a system and another two to get it up and running with the operating system and basic configuration. So four hours total, and we'll say double that for somebody with no experience doing it like my buddies ten year old grandson whom just built his first one, with minimal assistance from me. And even if we say a whole day, that is likely at lot less time than it takes to order something worthwhile even if you include the time to ship the parts to you. Sure, a lot of people have stores like Walmart and Best buy where you could just go buy a unit and take it home the same day but you will absolutely be getting a POS that way. So, the time argument is not really relevant. And that's not even counting the fact that like Karadjgne mentioned, practically ANY pre-built still needs to be gone through if you don't want problems.
2. No, it isn't dumb to equate intellect with the ability to built a system. Sure, the lack of knowledge might require you to go to somebody who has that knowledge to assist with the selection of parts, I don't know, someplace like HERE for example, but the actual assembly itself can easily be done by anybody that isn't a total lackwit and doesn't have four thumbs. If my 12 year old nephew and my buddies 10 year old grandson, both of whom aren't ever going to be rocket scientists, can assemble a system with minimal assistance, then any reasonably intelligent person can do it. That being said, having the ability to do it and having the desire to do it are understandably two different things. The fact is, most people could paint their own house if they wanted to. Most people just DON'T want to because it's tedious and because most people are too lazy to want to deal with it. Granted, it might not come out as perfect as when a professional does it, but then again, it might come out better because I happen to work in the remodeling/handyman/property maintenance trades, among other things, and I've seen what a lot of so called "professionals" turn out sometimes and it isn't any better than what the average person could do. Sometimes worse. Sometimes much worse. Even from some of the reputable companies.
It literally takes nothing more than a couple of screwdrivers and in some cases maybe a pair of pliers, to assemble a system. I absolutely don't believe that 90% of people couldn't assemble a computer with a little assistance AND that assistance could be a person or any of the MANY video tutorials out there that show you EXACTLY how to assemble one, step by step.
Your arguments don't seem very valid to me. I'm also not against buying a pre-built system so long as you understand that unless you buy from somebody like Dell you run a very high risk of needing to go through it because there are almost always assembly issues, and that with somebody like Dell you are about 98% likely to be getting something proprietary that will be lacking in some way and will be mostly unable to be upgraded or repaired without similarly proprietary Dell parts down the road.