Android MVP: login credential validation

I started learning MVP in the last couple of days and I found this interesting tutorial. I followed the code step by step to implement login functionality in my app. However, I still have difficulties understanding where the input validation should be! In the tutorial inside the LoginInteractorImpl class inside the login method (2 functionalities in one method!). My questions is, why would I have to reach the Interactor class to validate the credentials? Why not validating the input in the Presenter class and if the input is correct I pass it to the Interactor?