For the same reason that you never lower your margin. It's very easy to lower your margin, and you might get growth as a result of your lower margins, but it's a massive gamble with the most likely outcome being that you are now selling the same stuff for less money. And do you know what happens when you cut your marigns and don't see growth? You get fired.

Same thing with advertising. You drop your ad spend, you lose market share, and you're fired. There is no step 4. If you're right and the ad spend was unnecessary, you've made a tiny difference (no one cares about cost, only growth), if you're wrong, you're fired.