The name of the reason is: corporate rot. They don't have the organizational backbone that wouldn't let their "in-house manufacturing" rot away into inefficiency and waste.
Not that it has much to do with why automation fails to penetrate certain tasks. The reason why "long tail" tasks are often beyond automation is: piss poor ROI, calculated correctly.
You go out of your way to automate a certain process with traditional robotics, and it'll probably pay off in 15 years. The chassis this applies to is going to be in manufacturing for 10 years. At least half the systems work you've done there would have to be redone for the next chassis. Fun.
The bean counters counted their beans, and found out that using traditional robotics there is a losing game. Thus the search for better options. And the humans performing the tasks in the meanwhile.