Billions of people don't use calendar apps so they're useless; just remember your meetings.

Billions of people don't use todo list apps so they're useless; just remember what to do.

Billions of people don't use post-its apps so they're useless; just remember what you're going to write down.

Billions of people don't have cars; just walk.

You can dismiss any invention since industrial revolution with this logic.

Funnily enough at least in my personal anecdotic case it works about like that. I do just remember when my meetings will be (or look up where the meeting was decided on), do try to remember what I had planned (sometimes I forget, but almost always for the better), and written notes are rare enough that pen and paper are sufficient. And also don't have a driver license. I don't think my case is exactly rare, even among softdev croud.

The point, as I noted below, is that this is an impractical solution.

You can justify the value of any ridiculous invention by comparing it to a world-changing invention.

And I am pretty sure every single one of those "billions of people" have had the experience of returning back from the grocery store, only to realize they were actually out of eggs.