Why do criminals file the serial number off of guns in their possession?

I’m assuming that the gun is probably stolen, and they know it’s a crime to be found in possession of stolen goods. But there is always the chance law enforcement won’t check the serial number if it’s there in plain view. A police officer finding a gun with a deliberately obliterated s/n is going to instantly know he’s found a criminal. It’s a crime in and of itself to possess such a weapon. The person with the gun will definitely get arrested. So why try to wipe off the serial number?