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.
I've lived in both shoes on and shoes off households and I'm sorry, but shoes are filthy. I wipe my dogs paws when they comes in from walks and their paws are filthy and I wipe them every single time they come back in from walks. Shoes are never wiped.
Take a second, get a tissue, and go wipe the bottom of your shoe.
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.