Because (some) people like to get a "deal". And if you move your car brand to "no dicker stickers" and I don't, I'm likely to capture a share of the "likes a bargain" shoppers, though I admit I'll lose some of the "likes a sense of apparent fairness shoppers" to you.
Now, what happens when you have a "no dicker" policy and you need to move a car for whatever reason? Maybe the new models are coming out. Maybe the dealer needs to raise some cash to meet his loan payments. Maybe a model is intended for snow/foul weather usage, and you shipped too many of them into New England, expecting a normal winter. How do you entice customers who will eventually need a new car (according to them) into buying your car from your dealer now. Do you lower MSRP across the board for year-end models? That could work. Do you lower your MSRP only in New England? That could work, but smells worse from a globally consistent pricing point of view. Do you allow just the dealer who needs cash to lower his price? Well, you can't really do that and have any notion of consistent pricing.
And that's just the problems in the new car market, where a given car could be identical to any other any equivalent car. In the used car market, it gets way, way worse (in terms of complexity and sleaziness).