Oh please.
One of the MAIN themes of the game is the deconstruction of the "
Planet Of Hats" trend: every goddamn race presents different individuals.
Let's take the turians for instance: you defined them "militaristic". Let's take a look at their job descriptions, shall we?
There are turian mercenaries, turian Presidium Groundskeepers, turian barkeeps, turian merchants and more. Same goes for personalities: Saren was a homicidal ******, whereas Garrus is remarkably well-adjusted and morally upstanding.
Now, asari. Asari mercenaries, asari merchants, asari bartenders, asari vigilantes, asari crimelords: we've got all these and more. Think about that for a while and you'll see you can do that with all the other races. The short description you give (nerdy, militaristic, wise) is merely a stereotype that other races have of the race (man, I said race a lot of times).
And humans are regarded so highly because they're the next big thing: they arrived 30 years ago and they spread at an incredible rate while quickly advancing up the "food chain". It's not like they're presented as superior: much like in many other settings, they're the jack-of-all-trades race. They're not as strong as a krogan or as smart as a salarian or as wise as an asari (keep in mind, though, that we have indeed met our fair share of weak krogan, stupid salarians and above all idiotic asari), but they 've got a little bit of everything.
Also, do you even pay attention to the dialogues? Some aliens see humans as admirably skilled and special and others (maybe the majority) see them as vermin who spread alarmingly quickly.
Now, about humanoid races... first and foremost, look at AntiChri5's post. Bipedal and with two arms doesn't mean humanoid, no it doesn't. And even if it did, we've got hanar, elcor, volus, rachni, yahg, frickin'
Reapers, plenty of nonhumanoid species.
Other very valid reasons for the existence of humanoid aliens are:
* Convergent evolution. The "two arms, two legs" model is quite useful for sentient species.
* Programming. How the hell do you create weapons, armor, dialogue or even a 3D model for a gas particle alien?
* It's a videogame. Videogames are supposed to be fun, not realistic or reasonable. And, I, myself, think Mass Effect is ridiculously fun.
So stop torturing yourself with questions and enjoy the experience.
That is all.