You're always free to stop at the level of abstraction at which you find a certain answer to be satisfying, but you can also keep digging. Why are flat shoes better? Well, it's to do with my gait. Ok, but why is my gait like that? Something-something musculoskeletal. Why is my body that way? Something-something genetic. OK, but why is that? And so on.

Pursued far enough, any line of thought will reach something non-deterministic - or, simply, That's The Way It Is - however unsatisfying that is to those of us who crave straightforward answers. Like it or not, our ground truth as human beings ultimately rests on intuition. (Feel free to say, "No, it's physics", or "No, it's maths", but I'll ask you if you're doing those calculations in your head as you run!)