IMHO it is a good idea: https://en.wikipedia.org/wiki/Toilets_in_Japan#Toilet_slippe...
...huh. So that actually sounds internally consistent and weirdly sanitary. Like... Sure, the floor is dirty, but if you're permanently separated from the floor I guess that might be fine?
Would you feel comfortable lying down on a sidewalk and sleeping there, in a major city? Well, that's how I feel like when my friends walk around their houses with shoes, socks then jump on their couch. Their feet touches the couch which, to me, means couch == sidewalk. It's gross.
EDIT: Now, to be fair, hygiene is all about culture. I fully admit that. E.g. I seem to have no problem eating things off of a table in restaurants which is likely equally, if not more, unhygienic. But it doesn't bother me. Walking in my home with shoes on completely devastate me. Maybe it's a form of OCD, I don't know.
My family did it for years and some visitors had very extreme opinions about it, even though we never asked them to take their shoes off. Some got offended by me putting slippers.
So when I moved in with my then-girlfriend-now-wife, we made a rule... "it's our house so you take off your shoes".
We did warn him ahead of the rule, and told him, bring slippers if you don't want on your socks...
And yet, flip flops and sandals pass the test? Small issue with coherence.