I blame the dealers, at least somewhat. In the early days of the Scion brand, the idea was something like, all of the cars are shipped to the dealer as base models, customers could choose from a menu of add-ons, and the dealer would install them. Want just the basic car? Fine. Want to add keyless entry and mood lighting and a CD changer and chrome alloy wheels? They'll add those for you.
But (at least in my experience), that made for a worse product than having factory installation and QA. I bought a brand-new car from a Scion dealer in 2005 and indicated I wanted to add keyless entry. I paid the dealer, they did the install, and I left ... with a car that would intermittently fail to lock some doors with the key fob. I realized shortly thereafter that the dealer had installed an aftermarket system to save money rather than the offical Scion keyless entry system. I complained and eventually got them to install the right system, but jeez, that did not enhance my experience compared to just finding a car that was built in a factory with the options I wanted.
I'm not saying the modular Slate pickup isn't cool. I'm kind of tempted by it. But I wouldn't be surprised if people find themselves with leaky roofs, electrical gremlins and random squeaks and rattles compared to if they just bought some other truck/SUV and left it alone.
I vaguely remember that from the Scion days. I wonder what that does to liability? Like if it's something that impacts the safety of the vehicle - like a roll bar? - who takes on the liability if it's a dealer option?
Also assembly lines can achieve the scale factors needed to pay for all the jigs needed. A dealer doesn't sell enough cars to afford those expensive jigs. Making your cars in the factory is nearly always going to be cheaper the dealer customization. The dealer can do minor things that are quick and easy, but putting a different back on the bed on is something the factory should be able to do better and cheaper.