Because hosts can leave bad reviews of guests. This happened to us once when we were new to Airbnb, we had that exact thought: "We paid a cleaning fee, we're pretty tidy anyway let's just let them do their job." But we got a bad review and now almost every place we book, I get a phone call asking about the bad review and I have to explain... "We paid a cleaning fee, we're already very tidy people, and we were stressed and rushed getting our family with young kids out the door on time. The mess they're complaining about was probably 5 minutes of sweeping, but now we are even more meticulous when checking out."