The excuse usually made is "lead time for developing a car is years". Which is why I say commodity x64 hardware is the solution. Even the lower end Atom CPUs have more power than most of the OMAP based infotainment systems.
The busses are standardized, USB and PCIe aren't going anywhere soon, and SATA will remain for some time. Even if the underlying hardware changed (larger drives, faster CPU or new GPU) during development of a car, if they designed the UI and OS separate from the hardware it wouldn't mean a thing, except maybe the UI would be snappier.
Designing for a constrained hardware that's already obsolete at the time of the design choices is just a failure mode.
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2016 Granite Crystal Metallic TH, 3.2L, Cold Weather group, Tow group, Premium Leather group, 8.4AN HID Headlight upgrade Build date: 09/2015, BFG KO2 265/65/17, RRO Winch mount, RRO Super sliders.