Where are the open source car computer readers?

After having some car troubles this weekend I began wondering... Why do I have to take my car into a service shop and pay them to hook it into a computer for 5 minutes? Why do they charge to hook up to a computer for 5 minutes again? And if the computer in my hood knows something, and LCD readouts are dirt cheap, why can't the car companies put a small readout under the hood to tell me what is wrong? I'll never understand...

And why couldn't I just buy a little reader for $40 at AutoZone that could tell me the problem, AND I could reuse it, AND would be cheaper than the $50 - $100 that I'll be charged at a service shop? I'll never understand...