Why is million miles driven a better way to model it than billion miles driven, for example? If you happen to choose that, there clearly isn't anywhere near en

Why is million miles driven a better way to model it than billion miles driven, for example? If you happen to choose that, there clearly isn't anywhere near enough data. I'm honestly curious how one models this type of thing statistically, and I am not convinced enough of its obviousness to just accept numbers that someone throws around.