There are a few reasons:
1 - The companies that make car audio gear (Clarion, Alpine, Pioneer, etc.) put their focus into making gear that works in a vehicle moving 70mph down the freeway or driving through a pothole-ridden parking lot.
They are made to be more durable and provide a better listening experience under harsh conditions and in a poor acoustic environment. The money you pay for a quality headunit reflects the R&D these companies put into their car audio products.
2 - Manufacturers of mobile devices like the iPad/iPod/iPhone don't try to create stellar audio quality. That's not to say he audio quality from these devices is horrible, but it doesn't compare to a headunit, especially in the horrible acoustic environment that is your vehicle.
3 - The manufacturers of car audio gear don't want you changing their systems. They want the system to be as closed as possible, for a couple reasons I can think of:
- Changes to their system can alter audio quality properties, which would lower the quality of their product in consumers' eyes.
- They want the system to remain proprietary to keep consumers buying their products to work with their system.
For these reasons, I wouldn't expect to see a headunit based on Android (or any other mobile OS) in the near future, save for one that is heavily locked down.
If you honestly think that Alpine, Clarion, Pioneer and other car audio companies are selling you "junk", then I think you really should reevaluate your definition of "junk". (Now a crap brand like Dual - Yeah it's junk!)
Car audio hardware is designed with a function in mind, and they perform that function very well, as well as having a relatively trouble-free installation and user experience. You're not going to get either even with a pre-packaged carPC setup, as it's so vastly different.