Using your numbers, the US could supply itself for a fantastic 7 years and 8 months. And that’s supposing we cut all imports from Canada, Saudi Arabia, Venezuel... So the reality is that those 21 billion barrels would last quite some time. To the point where new technologies allow us to obtain oil from previously unreachable sources, which just keeps on happening.
Maybe you are of the mentality that if it only helps a little and doesn’t solve the entire problem, then it’s not worth making the effort.
Of course we also have the little problem of not having built a refinery since 1976.
True, but at what cost? Oil from shale ain't exactly cheap. It might (and probably is) more economical be to build nuclear power plants and wind mills and plug-in hybrids. Do the math and then come back.