This is because it requires more time and effort than subsequent visits. Sometimes it takes us twice as much time and efforts. Since it is not likely that it has been cleaned recently, a lot of dirt would have accumulated in different parts of your house. Besides, old dirt or stain is usually more difficult to remove than new ones.
Hidden and difficult areas to clean like under your sofa will definitely have so much dust. Besides, it has become a common trend that very old dirt usually gets mixed with grease and this makes cleaning more difficult and more time-consuming. But this usually won’t be the case subsequent times.