Looks like I misinterpreted the Volunteer part (I kinda shouldn't post 'round midnight, either). Still, him surviving the great heroic sacrifice part means it might come back in some plot later. Possibly, anyways, unless XCOM2 starts an entirely new continuity (which it might, if XCOM gets curbstomped).
I'm thinking a large-scale military attack is an easy target for aerial bombardment and the likes. And if a country (which, unlike XCOM, has a known location, has no subtlety, etc.) attacks the aliens in a coordinated fashion, the aliens can just send a larger amount of troops. Add to that that thin men can supposedly infiltrate human society and you've got a serious problem.
Don't forget how XCOM had trouble defending their own HQ, and that was a heavily fortified, secret base. Most national military HQs aren't that fortunate.
And yeah, mobilising even a platoon is a lengthy task. The Skyranger can get to a city in a few hours and by then it's already chaos and the amount of survivors is rather low already. By the time any military troop that can deploy ground forces gets there, it's likely there's nothing to save, just a few straggling aliens and a few very lucky citizens left.
On the aim thing...
Though the aliens don't move much during the turn-based abstraction, in real life, they would. And they move in strange ways, with strange patterns. Their physiology is strange as well. That's what leads to missed shots. (At least against thin men and sectoids) Against larger targets, it's probably the fear factor that that muton will get pissed if you shoot him and he's carrying a plasma rifle that can tear apart any modern armoured vehicle in a few shots. Ethereals have a psi-shield. Outsiders are made of energy, so they affect bullet flight paths. (This might also be a case where any alien power source is involved.)
Well, that's for ballistic and laser weapons, anyway. Plasma weapons are likely a bit hard to get used to for rookies. It also has blinding light effects which don't really help lay down much fire.
And there's the psychological effect of going against the vastly superior unknown as well. Sure, soldiers are hardened and prepared for it, but the complete unknown still unnerves them. They can understand what a human thinks and know what to expect, but not when it comes to aliens.