sunshinecorp: Because they don't taste good, dumb dumb!
WBGhiro: The hell you saying. Their meat is quite lean unlike guinea pigs and hamsters fatty texture, and unlike mice there's actually something on them other than bones.
It even says so in the wiki link i posted "During food rationing due to World War II, British biologists ate laboratory rat, creamed"
The key word here is "creamed". You can't eat it without cream, it's disgusting!
Laboratory rats may belong in the rat genome, by the way, but they're created for specific purposes through selective breeding. You wouldn't pick up a sewer rat and experiment on it. Well, you might. But it wouldn't be proper dinner.