Quite. Doubtless you've also not seen a commercial aircraft flight manual which specifically prohibits flight directly in to the face of a granite wall...yet somehow we all know it's a stupid thing to do.
The difference between a CB and a granite wall is that there's only one severity of granite wall. CBs come in a full range, from those in which the Bergeron process is just starting to produce rain, to those that break aeroplanes just for fun.
I'm truly amazed by the responses here -- I can only imagine that you all stay on the ground in spring and summer if you religiously avoid CBs. The key to safe flight is in avoiding the flight conditions associated with convective weather that are going to kill you. That means working out which CBs you can fly in and which you can't.