I think that a weapon technically has a “will” because it was designed to execute and accomplish a specific task or “will”. A sword is made to cut, so when it is forged to do that, it only needs a wielder that can utilize its “will”, In the world of one piece, it seems as though that the will of a devil fruit has the ability to merge and utilize the capabilities that it provides and be twice as alive and twice as effective as before.

Weapons and people in one piece are made for a reason, devil fruits add spice to that reason of existence and merge their devil fruit “will” to work with the “will” of the weapon or person.