In the live action demo that everyone has seen a dozen times, the guy walks amongst the giants, and only after he attacks their mammoth do they attack him.

I've encountered 4 giants, one being killed by some mercs.....but the other 3 have all attacked me on sight. Why?

Don't trust the demo. They're simply non-aggressive. They won't attack unless you get close.

i hoped to walk amongst them T.T

didnt they even say in the demo that they wouldnt attack unless you harm their beloved mammoth's?