I'm not really awar of many games -- be they board, field, or even word -- that don't have a winning, i.e., defeating the opposition, as the ultimate objective.
By comparison, IMO, what sets Diplomacy apart from other board games is that some sort of cooperative behavior is required to be successful -- at least typically. This unique aspect is like a repeating practical lesson in human psychology. Obviously, this "need for friends" can, and often is, exploited, and it is this "behavior" that I think the OP is commenting on.
One of the things that is always baffling to me, is the "stab to stab" mentality that some people play with. I understand that betraying trusted allies is a large part of the game, but I think some people find the thrill of stabbing to be too great to resist, even when it is easy to see (or should be easy to see) that doing so is of little to no benefit.
In chess, you never trust the opponent, so although it's thrilling to pull of a great fork, discovered check, or mate, it's expected and thus potentially less thrilling than a stab in Diplomacy. Personally, I get more of a thrill from alliance building than from stabbing. My business -- futures trading -- epitomizes the zero-sum game more so than almost any other -- every dollar that my firm makes is a dollar that someone else lost. I enjoy that aspect though, it's clean and pure in a way -- just like Diplomacy. It's not like some of these game Apps where you can just buy more units, grow strong and then go kick ass.