One reason: they make promises on behalf of the engineering team without their consent, in order to close sales. These promises become the engineering team's re

One reason: they make promises on behalf of the engineering team without their consent, in order to close sales. These promises become the engineering team's responsibility to manage from there. And the salesperson gets a bonus for closing the sale, instead of being punished as the engineering team is (by having to work overtime as a result, or by having to work on items that are less relevant to their career goals, etc.) Another reason: most engineering workers aren't given revenue sharing bonuses, but sales people are for selling the product of the engineering team's work, which breeds resentment. (A little like a line cook resenting tips given to a server, if not shared with the kitchen.)