Well, because without that energy to get the arrow moving the arrow will not have any momentum. An arrow that isn't moving has zero velocity and zero velocity means zero momentum. It would seem KE has a lot to do with bowhunting. It has a whole lot less to do with penetration than momentum but without KE the arrow isn't moving and there is no penetration and no bowhunting.
Not untrue ... but why do you can how much energy it take to get an arrow from not moving to XX FPS if it only hits and then penetrates 6" because of lack of momentum?
Again, the archery manufacturers have pulled amazing wool over bowhunters eyes here - because each FPS they increase their bows, they can advertise how much more KE they're producing - well duh, KE uses speed squared in the equation !
Imagine the extreme here ...
a 250 gr total weight going 400 FPS = 89 # KE
a 750 gr total weight going 200 FPS = 67# KE
which do you think penetrates better?
if KE rules - the 250 gr arrow would .... but nobody in their right minds hunts really big game with light arrows because THEY KNOW momentum is what carries the arrow and broadheads through.
KE is absolutely LOST when the arrow impacts because the equation itsself is only the measurement of the energy getting the arrow in motion !
its the weight of the total arrow that keeps the momentum going - that's why a high speen, light object it terrible at penetration and a heavy slower object excels at it !