Just FYI—Helmets don’t “absorb” energy, at least not in the way most people think. They dampen, disperse, and dissipate it. During a hit, the energy from the impact is primarily kinetic energy, which is the product of one-half the mass of the player, multiplied by the velocity squared (KE = 1/2 mv²). When two players collide, some of this kinetic energy is transformed into thermal energy as the materials in the helmet (or body) compresses and deforms. Some of the energy is also converted into mechanical energy (sound), which is why you hear the “pop.” If the helmets or players bodies deform during the hit, that’s also energy being transformed through deformation, meaning the force is spread out and thus reducing the impact on one specific location. This is the basis of impulse momentum theorem, which states that the force multiplied by the time of impact equals the change in momentum (FΔt = Δp). Extending the time of impact reduces the force experienced. In more elastic collisions, this energy remains mostly as kinetic.
The idea of leather helmets is ridiculous and regressive, and would lead to skull fractures.