I agree for the most part about even tablet apps not being "big buttons" but I think it's not quite as bad as you think.
Phones are a misleading example since the small size of the phone screen usually begs for small controls to fit as much info/functionality on one screen. I think most iPad-specific apps (or the iPad version of a dual platform app) usually have bigger controls and text than the iPhone specific version.
Plus, there's always the "zoom" accessibility feature or using an iPhone-specific app at 2x if possible. They're both cheesy ideas, sure, but the accessibility zoom is easy to use and basically magnifies everything for you.
Besides, for the most part we're only talking about basic apps you could reasonably use while driving, like Pandora, which really don't require that much button pushing.