Because you're limiting your graft potential with FUE alone. That, and the risk of transection is greatly increased. Where as FUT grafts are easily dissected.
Enjoy your 5,000-6,000 graft potential with FUE
Where as though who maximize FUT, and finish with FUE end up having 8000-10000 on average
If you do the math, FUE produces more scaring then a fine linear line shown from a FUT procedure. Essentially if you add up all the dots, you have a scar that resembles a baseball on the back of your head.
A good FUT surgeon will leave a pencil line on the back of your head that can be easily cut to a #2 guard