The first question
Money is a both a good and a method of exchange. As a good, it has a limited supply, and therefore there is a demand for it. There is a demand because people can use the money to purchase the goods and services they need and want.
Therefore money is treated like like any other goods whose value is determined by the forces of demand and supply.

To the second question;
why government’s don’t print more money to deal with the problem of national debt:
The reason is that printing more money doesn’t increase economic output in any way – it merely causes inflation.