There are asteroids with concentrations of precious metals more valuable than earth's entire economy. Why don't we just send up spaceships to mine them and send the haul back to earth? What country would say no to free money?

After all, it's just an engineering challenge, not impossible.

The numbers on that are at least somewhat questionable. Even ignoring that you'd crash the market (thus it's not actually worth what it first appears to be) what is the total fuel cost to adjust the orbit of the target asteroid to land the entire thing back on the earth? Because that's what you're doing bit by bit as you shuttle loads of ore back.

Now if you have space based manufacturing or fuel production on the other hand ...