Have you ever heard of the Pitch Drop Experiment? That's what comes out of tar sands. Perhaps you can see from the picture why its so difficult to turn into fuels like gasoline.

The term "gold into lead" might, at least partially, refer to the fact that this stuff can be used to make plastics and some other useful materials that are of enourmous value to modern society (especially modern medicine). Putting so much effort into turning it into fuel really is rather silly.

rainsong is right to say that the viscosity of bitumen has to do with the length of the polymer molecule in it. For polymers, the longer the molecules, the harder they are to pull apart from eachother.