Because there's isn't really an argument innit - at least none that I took notice of. Isn't it just exploring the reasons why it is like it is today? They even made it abundantly clear in the beginning (and in the comments here) that the rating is fine for the app

And for what conceivable reason would this need to have sure underage people aren't using it?

A period tracker has relevance in the context of a sexual relationship, but there is really nothing about it that needs to be censored from underage people. It is not explicit content. It's a specialized journal, that's it