Why is that mages always attack on first sight? I mean, not even Hagravens attack on first sight. Hell! Not even Giants attack on first sight.

Hagravens do attack on sight.
I've walked right up to necromancers and they just warn me.

Not all of them attack you right away. In fact, the one at the ritual stone left me alone yesterday when I walked up there. Well he left me alone until I ran up and power attacked him, anyway.....

enemies except few always gives a warning even animals gives warning hagraven never gives warning if you are in their reach they will attack you on sight

I can walk alongside a giant with a mammoth and he won't attack.

they will if you will trespass in their territory ( near their camps ) otherwise they are cool creatures never hits player unless provoked

Even forsworn will warn you away, pretty much hagravens are the only ones who don't.

Because they don't have a very good defence and want to get as much damage as possible in before you get close!