Transfers are not necessarily only taken to refill from those who leave before graduation. Some colleges’ business models are designed around significant numbers of transfer students (e.g. Cornell, as well as many state universities), while others’ are designed around no or very few transfer students (e.g. the military service academies where all students start as frosh, even if they have post-high-school college credit, and some colleges like Princeton and Stanford that take only a few dozen transfers per year).