I'm not sure what perspective you're coming from (are you still in high school?), but /u/Aclopolipse gave you the correct answer. The number of years a student attends the school has marginal impact on the total revenue generated by tuition.

College isn't structured like high school, where the four years are clearly demarcated, and everyone in each grade more or less takes the same classes (e.g., all freshmen take English I, all sophomores take English II, and so on). With very few exceptions, students can take classes whichever year they want, provided they satisfy the prerequisites for all the classes. So a math major might be in Calc 2 or freshman year, but someone who is only minoring in math might take it senior year. And for some majors, a student can cram all their classes for their major in the last three or four semesters, even the intro classes.

What all this means is that the cost of providing a college education is not broken down by class year, but rather by department. Whether the school has an abundance of juniors vs. freshman makes little difference to the tuition generated by the students -- the school makes the same amount of money if they have 1000 freshmen and 2000 juniors, or 1500 of each. Yes, the school will only be able to charge those students for two years, but then all the school has to do is replace those students two years later with more transfers. There is no monetary loss.

The only real variable here I can think of is room and board -- at some schools, upperclassmen are less likely to live in a dorm. But then again, a transfer student might actually be more willing to live in a dorm than other upperclassmen, since he or she is less familiar with the campus, so that might be a factor too.