Why don't governments mine?

I was just wasting my time on the internet and my daily life like usual and asked why don't they put the super computers to work? You know they cost a lot of money to build and don't really get any true return for them with money wise so why not just mine with it? Think about they could have the largest control of the Bitcoins and make the currency worthless and stop anyone's plan of having an internet currency all together. This would mean the old way of currency will stand. This is just my random thought I had through out my day.