I think you're assuming too much here.
VGA might be dated, but it works perfectly for in the car. HDMI cables are friction fit, good luck keeping those in on a bumpy road. They WILL slide out. Not to mention there simply is no need for an HDCP compliant connection, it's not like you are watching bluray movies while driving. Not to mention HDCP compliancy driving up the cost of the monitor itself. VGA/DVI adapters add at least another inch and a half to the space required behind equipment. In a car, that's valuable mounting space!