I genuinely don't understand why cars suddenly
  have to have tablets built into them.

Back in the 1990s, in-dash sat nav was a premium feature only available on high-end cars. The first all-in-one TomTom wasn't introduced until 2004, and before that people in sales were willing to pay a big premium for sat nav, as a business expense.

Hence, car companies _think_ an executive/luxury car needs to have a screen in the dashboard because that's just how it's been for as long as they can remember. And they think putting premium car features in midrange cars makes them more appealing to buyers.

In the US, all new built cars are required to have reversing cameras from this month, so they need some screen in them. Once you have that there, it makes a fair amount of sense to start to use it for other functionality when not reversing and to use it to replace the old media interface.