marketing's incessant notion that lying to people is like a science of making money just makes me sad. make a good product, be honest about it, change plans if noone cares, what's so bad about that?
Competition. If you don't do it, you'll get outcompeted by those who do. It's not true in many areas (where the market isn't oversaturated), but that's the perception - so people go and blatantly lie even if they don't have to.
It's a kind of problem that should be solved by social norms and regulations. They are way too lenient right now, and the continued acceptance of advertising as a respectable occupation perplexes me.