Why is it that the U.S. would even consider selling $20 billion worth of weapons to Saudi Arabia?

It's called greed, which is human nature. Get the $$ today and worry about the it later when the s$it hits the fan. Why weren't the levees in New Orleans ever fixed before Katrina? Same logic, the government knew the possibilities of the pending disaster but rather than be pro-active our leaders are re-active and keep their heads buried in the sand until it's to late and they have to spend many times more to fix the problem afterwords.