These things are terrible at finding the causes of back pain. But that doesn’t mean it’s not there.
However, just because we can’t seem to reliably find smoking gun causes of back pain with modern imaging technology does not mean it isn’t there. Physical therapist and researcher Tony Ingram makes this important point in a short, clear, evidence-based opinion piece [content no longer available]. See also this academic paper from 2011: What happened to the ‘bio’ in the bio-psycho-social model of low back pain?