Well, it's a strange issue. Without such testing, they wouldn't exist at all. So, is a life of, maybe okay but maybe awful lab testing better than none at all?

On a human level, we generally value being alive no matter the conditions, e.g., debilitating illness, chronic pain, etc., over not being alive. Of course there are exceptions like euthanasia, which are relatively rare. So why not apply that ethical equation to lab animals as well?

I'm just hypothesizing here. I don't particularly like animal testing, think its cruel, think it may be necessary, think it's a morally ambiguous situation, etc. Which is to say it's a complicated topic.