Is it normal my shoes get nasty and smell terrible

How do other people wear the kind of slip on closed shoes that you wear without socks.... Without them getting very smelly and dirty

My shoes have a layer of black gunk on the inside from the accumulated sweat dirt and whatever else. My shoes STINK, my feet do too as a result

I dont know how other people do it without it being this bad. Once again i am referring to the kind of closed slip on shoes that you do not wear socks with