Why do dogs sniff so much?

Dogs have a terrific sense of smell. It is one the main ways they communicate with each other. They like to smell who has been here recently and also let other dogs know that " hey I've been here too." I never saw a dog that didn't sniff and smell on the walk.

Assuming you sit pets quite frequently, their scents linger on the floors, in the furniture, etc. Unless you completely disinfect and deodorize between each stay (which still isn't guaranteed), a dog's keen sense of smell will lead them to be curious about who has previously marked territory in your home. I wouldn't be concerned about this behavior.