True. But most humans are quite a bit less mass than a mammoth. And the physiology of humans (the fact that their limbs are almost as long as their torso) makes it much easier to freeze.
Yes, but mammoths were able to live in a climate where humans could not last long without clothing. Besides, what was going to eat a dead mammoth? Small mammals like rats, foxes, etc. would find it hard to survive in the climate where mammoths could live.