BobK enlightened us all on that.
When people are learning English, they have to learn when to use 'are' and when to use 'is'. 'ain't' is not so much a contraction as a fusion of the two, which then has universal application. Voilą! I speaka da English.
I am : I ain't
he is : he ain't
you are : you ain't
they are : they ain't