The whole point of making a character is to create your own goals, killing monsters may be an ends to said goal but I don't see the default goal being kill monsters and take their stuff in D&D...
If killing monsters is a means to achieve the character's goal, then optimising for killing monsters is optimising to achieve those goals. That just makes it a sub-goal, but it's still rational for the character to optimise for it.
Otherwise why a Diploomacy skill? Why feats that have no combat application like linguist?
OD&D - and everything up to the original Oriental Adventures for AD&D, in fact - didn't have anything similar to "Diplomacy skill" or "linguist feats". Even in the later editions they are tacked-on elemets with no proper systems support; the closest it gets is 4e's "Skill Challenges", and I'd hardly call that "comprehensive support". The primary systems support in D&D is and has always been for killing things and taking their stuff; that is the basis on which I say it is the "default" or "vanilla" expected goal (or sub-goal).
I think you're projecting that goal as the default goal of D&D?
No, I don't think so - I'm looking at what the D&D systems support and inferring from that what the main focus of play is expected to be. Of course, some folk might deliberately go against expectations, but going against expectations and then complaining because your goal is not that best supported seems to me to be quite bizarre behaviour (not to mention remarkably self-centred).
SOO then you've answered neonchameleon's question right here... the goal of every character is not to optimize to kill monsters.
What is "vanilla" D&D?... and no, again character's exist for the reason that a player chooses for them to exist... that's always been a part of D&D.
Taking these two comments together, the idea of roleplaying games in general
is indeed that the player may select any goal they choose for their character. But systems are tools - they may be used to structure and determine how resolutions are made for a roleplaying game. It would seem sensible to select a system that supports the sort of resolutions that would be expected when pursuing the goals the players have set for their characters. D&D primarily supports resolutions concerned with killing things and taking their stuff; a large proportion of the rulebooks deals with resolutions of just this sort. It seems to me sensible to select D&D as your system when the goals the players choose would be expected to be pursued by killing things and taking their stuff. If all or most of the players choose very different goals, it makes much more sense to use another system - one more suited to the goals selected. Not to say that some "twiddling around the edges" can't be accomodated quite easily, but if every player in the group wants to have goals of peace and prosperity to all gentlebeings then I would be selecting another game system than D&D to run the campaign by...
Killing things can be the goal and/or a way to accomplish said goals but the default, IMO, is whatever you want your character's goal to be. This, IMO, is very much a playstyle thing as opposed to something the game can set for you.
My experience is that character goals are best selected in one of two ways; either:
1) The system is selected and then players select character goals, the expected routes to achieving which are supported well by the ruleset chosen, or
2) The players, preferably in collusion (since roleplaying when the "team" of characters all have highly disparate goals tends to be an exercise in frustration) select character goals of their choosing. A system is then selected based on what system is thought best to support the expected means of achieving those goals.
Selecting a system and then having players select goals that are poorly supported (or completely unsupported) by the mandated system leads to very poor play experiences, IME. That hasn't, apparently, stopped people from trying, but there it is. When they insist on trying, find it an unsatisfactory experience and then demand that the system needs to be changed to fit their chosen goals, however, my gut response is usually in the "roll my eyes or get testy at their short-sighted selfishness" region.