Because there is a need for redundancy with effing with an effer.
person 2: Fuck You too. FUUU2
person 1: Fuck You so fucking much. FUSFMMMMM
person 2: Fuck you so fucking mucher
person 1: Fuck you so fucking muchest
person 2: fuck you so fucking most!
person 1: Fuck you so fucking mostest
person 2: Fuck you so fucking mostestest
person 1: FUCK YOU SO FUCKING MOSTESTEST!!!!!! that's in capslock and with dozens of exclamation points. i win.
person 2: darn it.