Because there is a screen that displays both the current speed and things like turn by turn directions. If you somehow crash that screen, you lose both (for a minute or two)

So some genius somewhere thought having a single screen was more important than keeping basic functionality like speedo and turn signals safe from <checks notes> anything that the cars web browser might be exposed to.

I mean that sounds reasonable, it’s not like a web browser has ever been compromised before.

In industry, it’s usually referred to as “infotainment”. That’s why the information display (speedo) is logically linked to the entertainment (radio)