Just think this way, it is retail store and the salesman your talking gets paid by minimum wage or maybe a little more and has limited knowledge. No retailers would want pay more for somebody expert in something but mainly trying to sell stuff.
#1 I have to disagree that it is cheaper to build your own computer. They both are basically the same. I would agree if we are still in 20th century. I do build computer and still know how to build them.
#2, the PC market is now dying! It is fact, less people want to buy desktop nowadays, hence referring to #1 above. Micro$oft isn't doing great with OS anymore, and they are making worse. My son asked me to upgrade his computer. I checked and it is not worth the investment and I actually checked whats available off the shelve or just build one and I ended up get same cost. WTF?
#3 Anything wrong with that fruit that got a bite by Steve Jobs?