Because you still have to co-exist with other software beyond the type system's boundaries. The amounts are going to be converted to/from JSON, put in and out

Because you still have to co-exist with other software beyond the type system's boundaries. The amounts are going to be converted to/from JSON, put in and out of databases, show up in logs etc. When it comes time to invoke Amount.fromCents(..) at the boundary of the system, it's nice if the front-end posts a variable called amountInCents.