Because they are paying big money to get airtime.

The MOST important thing that they are trying to accomplish is establishing name recognition. Nobody really remembers the rest of the commercial.

Why would they give more name recognition to their competitor for free?

(Name recognition. The effect that when you go to think of a product category, it's what pops into your mind first? Quick: think of an electric car company. Whatever you thought of - name recognition is why)