There's nothing unsanitary about it. The bottoms of your shoes simply are not that dirty unless you're walking through abnormally dirty situations.
Like outside? It's so wild to me how some can step into their living space (or even bedroom) with the same shoes they use outside.
That's the whole function of shoes though. If outside was as clean as my floors at home, I wouldn't need shoes.
I have to imagine the person who leaves their shoes on either never goes outside, or just has filthy floors.