There is tons of inventory of complete black everythings right now. When you can buy a S&W M&P-15 M-4gery for $800 and get $100 rebate why build? I think Sportsman's Warehouse had Bushy ORs for $700 during a sale this summer. Most people buy stock M-4gerys and maybe put a light and Eotech on it. Most people don't have the knowledge or skill to build an AR, and don't want to. And the big reason that ARs are cheap: most people that want one bought it during the first year of the O-scare, and have no desire for more ARs, or money left to spend.
Parts normally cost more than a whole thing. Buy a car; buy the parts to make a car. Parts cost more; a lot more. Ordering, receiving, inventory, pulling, packing, shipping all make parts cost more. Do all those things for 10,000 parts (and make 10,000 little profits), versus 1/100 the work for 100 guns/cars/illudium Q-36 explosive space modulators. I would charge more for the parts too, because they would cost me more as a business. That holds true if I'm the manufacturer or the retailer.
Taliv, I have been watching parts prices too. DSA uppers started out a year ago at $60, but as you say prices have risen. Didn't see $60 stripped uppers at Palmetto and Denny's but $70+. BCM has a great deal on uppers right now IF you don't care about perfect appearance. They have stripped uppers with FA and ejection port kits for $70.
Some of the BCM barreled uppers are on sale for cheap too. I am trying to save enough for the mid-length lightweight 16" and add their BCG. For BCM quality they are cheap. DSA has even cheaper barreled uppers. DPMS had a screaming deal on complete uppers at Labor Day. Get on the email lists for most of these companies and you get earliest notice of sales they have.
Both Brownells and Midway are out of the DD 4150/CL M4 barrel that they have been selling for $169 all year, but Midway says it will backorder. PK Firearms has barrels and parts for OK pricing.