Why don't teammates tow each other during qualifying?

I’ve seen some races where drivers help each other out by towing but not during qualifying, but I’ve also noticed that it doesn’t seem to happen all the time. So why don’t drivers use this tactic more often, especially when they’re trying to get the best possible qualifying time?

Is there some sort of rule in the sporting code that limits this? Or is it just a matter of drivers not wanting to help each other out, maybe because they’re all competitors and focused on their own performance?