Why are people so quick to mock an enemy based on appearance or other, adjacent trait that is beyond their control?

If that's your go-to attack vector, you might want to reassess. Attack their values, their integrity, their actions, their intent, or their judgment instead.

Attacking things beyond their control is attacking what it means to be human. No one wants to be 80 years old. Or to be unattractive, or have a physical characteristic that draws attention or is insufficient in some way.

What purpose is served by pointing these things out that are beyond someone's ability to change?