I'm not sure they don't, but I'll grant you that it's open to debate. Nobody does supply chain better than Apple, though, and that's gotta be worth something.
If they wanted to Foxconn it, I would think a company like Kia or Hyundai would be a logical partner for them to get into the business. Apple designs it with the help of their manufacturing expertise and imposes their usual tyrannical quality requirements (maybe with the help of some employees they could score from Honda or Toyota to get the requisite quality levels), their Korean partner produces it and everybody's happy.
And what would the benefit be to Kia or Hyundai. These firms have already entered the market chasing Toyota and GM. They have their margin structure baked in already. Moving to low-margin/high-volume contract work would cost them a ton of money.
It's the same reason that, say, Intel doesn't (can't) do consumer logic fabrication to compete with TSMC or Samsung. They have the technical know-how, but their fab capacity is already filled with Xeons that fetch like 4x the margin.
Your main point is valid in the contemporary sense, but don't forget that the Silicon Valley is called Silicon because it used to be the global fab hotspot. While manufacturing chips, boards, accessories, computers, hard drives, card readers, and mainframe racks isn't quite the same as cars, there's plenty of manufacturing know-how in the Silicon Valley. Does Apple have that kind of know-how, and does it make sense for them to acquire it? Not so clear.
whoa...chill out on the kool-aid