Because they lie. Directly, or through omission, or through implication, or through some ridiculously convoluted means, but we all know it to be true; we cannot trust the advertiser.
> we cannot trust the advertiser.
Of course you can't. That's why you don't just take advertising at its word. Anyone who does is a fool. But to say that you shouldn't allow comparative advertising because of the possibility of lying is absurd. There are other laws that cover truth in advertising.
Using someone else's brand, without their permission, in a way that will damage their brand, to try to sell your own goods, in a way that everyone knows is going to be fantastically biased. Seems pretty out of order to me. :)
Seems more direct than the alternative. If you believe you have strengths compared to your major competitor, why is it bad to lay out the comparison explicitly in your marketing? Sure, comparative advertising can go to far, but why not leave it up the company doing the advertising to make sure they don't come off looking petty?