I am prepared to hear that IABU as:
a) I hear this expression all the time and
b) I've never heard of anyone else be irritated by it!
...but, AIBU to find the expression 'Why can't you/we just...' a bit patronising? There's a guy at work that says this ALL this time, often in relation to things he doesn't know much about.
In fact, now I think of it, more often than not, when I hear this expression, the speaker is usually suggesting an overly simple solution to a problem, which I've already thought of and discounted/taken into account.
And even if the person has a point that's not been thought of... I think I usually would say something like, "Ah ok, so the system doesn't allow us to do X. That's a shame".
Or am I just being prickly?