They're like "default allow" network security. You can only catch the nasties that you've seen before and explicitly tell the system what symptoms to look for. And with the diagnostics, they also tend to assume digital failure modes: it's either working or it's not. They don't deal well with cases like yours of intermittent functioning.

Readers' Digest did a story a while ago where they took a thoroughly-checked car to dozens of different mechanics and service stations. The only problem was that they loosened one of the spark-plug wires just before driving in, so it was occasionally arcing. At one station, the 19-year-old mechanic hooked up the computer, waited five minutes for the result -- without ever looking under the hood -- and pronounced them in need of a new crankshaft. Hey, it's what the computer said.