We have had viruses leaked by other countries in the past, with US and Germany coming to mind. If I remember right it was around 2005-2010 when democratic nations officially started to state that they had a right to use malware against criminals and military targets, and from that on it has just kept going. As I recall, Bruce Schneier and FSF made a bit of articles and talks where they tried to get people to understand that once democratic nations starts to use malware offensively they also generally stop protecting their citizens from those security holes.

That we don't hear about it more often seems to me that nations that develop this stuff seem to have gotten better in making malware more precise in what they target and less leaky to the general population, but it could also be that media just don't report about it as much. When a gangs anonymous chat get hacked, or a tor hidden service website get taken down, news seems more interested in talking about the crime than what security vulnerability that the police used.