There can be many reasons why the cost of government is high, depending on your political and economic beliefs.
The first reason is that people want the government to do many things. The government is expected to protect the people from crime, to prevent people from becoming too poor, and many other things. This costs money.
A second reason is that government is (to conservatives, especially) inherently inefficient. The government will spend a lot of money because it has not competition. The government does not have to be careful with its spending because it can't go out of business.
A third reason is a mixture of these two. Governmental agencies (or bureaucracies) always spend as much as they can. By doing so, they justify their existence and make it more likely that they will keep on being funded.
These are a few of the many reasons why the cost of government is high.