A $300 tablet drops support in 90days to a year. A $300 tablet equivalent in a car has a trailing maintenance requirement for service and parts for years (5-10?)

A 300 dollar tablet dies quickly in the automotive environment with outdoors-like variation in temperature and humidity. Tablet manufacturers literally write in their product sheets "do not leave the device in a car or it may get damaged"; if you leave an iPad in direct sun for a month and it dies, it's expensive for you, if the same happens to a car console, that's expensive for the manufacturer which has to repair it under warranty.

The temperature range for iPad listed by Apple is 0 to +35 C for operation and -20 to +45 C for storage. The required temperature range for electronics on automotive dashboards (which may be exposed to direct sun) is -40 to 90 C, which effectively requires different materials, which means you can't even reuse most components.

assuming you're selling 5 million of a part where the previous generation is 5-10 years old, $1.5 billion is not a lot of money for development, validation, materials, manufacturing, logistics, dealer training, integration with future models, and a decade of OTA updates

Because it becomes a $3000 tablet, in today's chip-shortage environment, to meet certification for automotive use.