first off, I just want to say that I laughed at Bob Loblaw's answer for about a minute. That was hilarious dude.
Anyhoo, I don't think they are intentionally designed that way, it just isn't a priority. It's not like soldiers are the ones buying the vehicles so there's no incentive to make them comfortable. There is a lot of incentive though to make them as efficient as possible so if comfort has to be sacrificed for that, then that's the way it goes.
Also, why the hell do you need padding on the walls if you're in body armor and have your kevlar on? Even the old hummers had heat, a/c is kind of hit or miss. I've never flown a helicopter so I have no idea on that one, but I've heard of some that do (for all the people making fun of him for that, the blades eject too when you eject). I'm sure there is some reason behind not having them though. If you are going to loose the chopper anyways you might as well save the person flying it because they cost a good bit of money to train.
Anyways, they aren't all that bad.