Counterfeiting is not a minor offense. It is not like running a red light, or even shoplifting. These crimes are misdemeanors handled at the local level by local police and courts. Counterfeiting, on the other hand, is a federal felony handled by the U.S. Secret Service. In fact, the entire reason the Secret Service was originally created was to handle counterfeiters. Only later did it take on the role of additionally protecting the president and other key government officials.
The reason why counterfeiting is treated as such a serious crime is because money is so important to our society. Money is the oxygen of the economy. If people cannot trust that the money they carry is authentic, then it gets much harder to buy things and the economy slows down.
When you get caught counterfeiting -- and if you are counterfeiting you will get caught eventually -- the punishment can be unbelievably harsh. According to this article from Wired Magazine, entitled "Junior Mints":
If up to 15 years in prison is not enough to deter you, there are additional penalties as well. Authorities will also seize any property used to create or pass the bills. That means your computer, printer and scanner will be confiscated. You will also have to pay restitution and probably a number of fines, too.
In other words, the picture is pretty clear. When you create counterfeit bills, you will get caught. There are a ton of people out there looking for fake bills because they don't want to get ripped off. When you get caught, the penalties will be severe, and it's likely you will end up spending some time in jail. You will have a federal felony arrest on your record, making it very hard to get a job.
In other words, you can't get something for nothing, at least not if you are trying to do it by counterfeiting. Although it is easy to print your own money, it never works when you try spending it.