The computers are, by far, the most reliable parts of a car. They’re not subject to mechanical stresses or wear, and the real-time/embedded operating systems are far more fault resistant than desktop/phone OSes. The computers also mean that you can buy a $20 OBDII scanner and have the car tell you what’s wrong with it. Maybe an extra $10 for an app that will decode most of the manufacturer-specific codes. The difference between those $30 diagnostics and the $10,000 system the dealer uses is mostly that the dealer system includes all the manufacturer codes and step-by-step directions for fixing each fault.