If car manufacturers would decide to support upgrading the Qt version by uploading shared objects directly to the car there would be no need for them to buy Qt Licenses. Because technically they could support all points of the LGPL. But that will never happen for two reasons:
1) It is very hard to get out of the contract if you already bought a license. You would have to prove that you aren't using any commercial tools anymore (of course they are the same tools as GPL but installed a different way).
2) They are car manufacturers and known to lock down their systems as much as possible.
The Qt Company probably didn't think about the average C++ Developer or Desktop programs at all while writing these Terms and Conditions...