While you statement is True it's misleading : Frames may be made by just a few Suppliers : But how things are machined and fitted is what makes the differences : How the crank and Forks fit as well as line up all add up to the differences in Cheap ,Good Quality and Top of the line.
While your statement is also true, the examples you gave aren't very relevant. For instance, cranks attach to the motor, not the frame. Has anyone heard of the frame motor mount misalignment issue, or even a frame-fork bearing misalignment issue? If they have, they're very rare.
That's not to say there aren't differences in quality control. The same manufacturing line in China/Taiwan/etc can produce the same items at different quality levels, depending on how you spec and how you enforce (do you have your own QA people on-site at the factory?). Where you're likely to find differences in frames is in the quality of the welds or consistency of the materials.
Similarly, the guitar example isn't apples to apples. You might as well compare a Specialized to a Seven. The Seven is a hand-built frame designed specifically around you, your measurements and your riding position. Specialized are mass-produced, not hand-built like a couture manufacturer like Seven.