Plants need to be repotted because not only does the nutrient quality of medium deteriorate with age, but the structural qualities as well. Soil compacts and loses it's ability to hold air and water. It also fills with dead and decaying root matter. In the wild, things like annual plants and tree roots work to expand and refresh the soil, storms wash up the topsoil and mix it with newly dead plant matter, and creatures dig burrows and forage just below the surface. All of this yields a very diverse strata for wild plants to grow in. In a pot, you only have a few plants, and typically they deplete the soil in similar ways, so the soil must be changed regularly or appropriate associate plants must be added.
~Joe