Why would a small company get away with infringing on a patent for 20 years?

I am trying to understand the concept of patent trolling and how a person or small company can secure a patent and then get away with doing nothing with it and infringing on it until they are sued. Why did the patent office allow them to have the patent in the first place?<br><br>In the US, a patent is good for 20 years from the date it was filed. If a company has a patent and isn't using it, they should be forced to license it out to other companies if they are infringing on it. Pretty simple concept to me.<br><br>I just do not understand why the government allows patent trolling and then allows people to sue companies over it and not make them license the patent out.