In reading about the now infamous hacks on cars these days  it begs the question for me - why is this even remotely possible?

Why does a car need to be built in such a way that the brake pedal or steering wheel or any other driving component can be operated by the computer? I understand this may be helpful for diagnostics but surely the risk outweighs the benefit?

If, for some reason I cannot fathom, these capabilities must be in a car these days why is the entertainment and internet capable side of the system not air-gapped from the critical systems? Surely it's not hard to design things so these kind of hacks will be impossible.

Am I missing something here?