Because it wouldn't be a challenge if you just got the input as an array and had to return a tuple.

This kata is about learning and using string operations. This is the learning goal. Why do you need these string operations? Imagine you write a small programm that reads an input number and squares the number. In many languages, this includes Python, you will receive a string as an input from the user. So it's important for you to know what to do if you receive a string while wanting to do some math with the input.