Why does an optional property add a " | undefined" instead of just being an optional property?

const schema = type({
  'name?': 'string',
  email: 'string.email'
});


// typeof schema.infer
//  {
//     email: string;
//     name?: string | undefined;
// }

type WhatIWanted = {
  name?: string;
  email: string;
}