Fragmenting bullets increase the chance of hitting something vital. Unless a vital organ is significantly damages, the mechanism of death is typically blood loss. BHowever, vertain organs are particularly vulnerable to damage from high energy rounds (the liver comes to mind). These organs can be severely compromised by transferring a lot of energy in a short interval. This is best accomplish by a bullet that expands, tumbles or fragments.
BTW, bullet yaw or upset achieves similar effects to expanding bullets. A kawing bullet has it effective frontal cross section increased to the length of the projectile. Stable bullets drill right on through.
Finally, which a deer is similar in size, they - and most wild animals - are in far better 'shape' than the typical human. Physical fitness has a great deal to do with the ability to absorb punishment and resist shock (although there is no straight correlation)