Because of the ways that violence works.
It's rarely about the violence itself. Violence is always, always a means. It's not always rational or whatever, but violence always has a motive.
Go aggro is for when you care about what they do; whereas, seize is for when you care what they have. Violence is already on the table.
Well, it means that if they call your bluff you're gonna have to decided how much you don't want them dead.
You also get situations where hot characters are more intimidating than hard characters, and that on a hit of any kind you get what you want without them being able to 'suck it up'. I don't really care for seize as a basic move, but that's already a beaten horse.
Yes, they can't do stuff for you if they're dead...
Wait, about bluffing: We have always played it so, if they force your hand, you have no choice: You have to
pull the trigger/lever/whatever. Going aggro is putting yourself and someone else in an irreversible do or die situation...