It isn’t clear why Google would want to be the steward of the “Half-Life” video game franchise or be the company that sells to the world the “Left4Dead” zombie-killing game. But rumors are floating that the search giant is buying Valve LLC, the Bellevue, Wash.,-based game developer.

[Update: A source close to the matter tells us this isn’t going to happen.]

The reason is that Valve owns Steam, the premier digital distribution network for computer games. Valve has become one of the biggest purveyors of games through Steam, which has more than 15 million PC users playing its games. Steam is a way to distribute and facilitate the play of high-quality online games such as Valve’s Counter-Strike team-combat game. But it’s also an end-run around retail stores and a way to put an end on piracy through online security controls.

Valve is best known for its games such as “Half-Life 2,” “Counter-Strike,” and “The Orange Box” (featuring “Portal”) — all of which have sold millions of units. It is headed by Gabe Newell, a former Microsoft executive who founded the company. The attraction for Google is that it could become a leader of digital distribution and expand Valve’s technology so that it can be an e-commerce vendor of all sorts of digital goods.

A spokesman for Google declined to comment. But Valve spokesman Doug Lombardi said in an email, “I wonder when Google’s going to call and tell us.”