Has anyone ever wondered about the three blind mice?

Three blind mice, three blind mice,
See how they run. See how they run.
They all ran after the farmer's wife.
She cut off their tails with a carving knife.
Did you ever see such a sight in your life
As three blind mice?

Why is the focus on their blindness and not their taillessness? On their inborn disability rather than the disability caused by violence? Is the idea that it's worse to be born disabled than become disabled?