I think it is for the same reason that many STEM programs require their undergraduate students to "apply for entry" to the STEM part of the program. For exampl

I think it is for the same reason that many STEM programs require their undergraduate students to "apply for entry" to the STEM part of the program. For example, at $GenericUniversity all CS students are listed as pre-engineering until completing calculus, data structures, discrete math, and a few introduction to programming courses. The school then "weeds" out students by making the initial coursework high-paced and somewhat difficult. After each round of courses a few "weaker" students drop out of the program. Eventually, the students that make it through the pre-engineering curriculum apply for the engineering school and are admitted, assuming they have the right grades. The school now hopes that the students who have made it this far will be be the "better" students and thus will get better grades and be more likely to graduate. By weeding out the "weak" students before the students are in the Engineering School, the school can report a higher graduation rate and overall GPA for students in the program. Though everyone knows that many engineering students failed (or were failed by) the program, it doesn't go into the School of Engineering's reported stats. I don't have any sources on this, but I do have numerous friends who work for universities and have heard this same story in varying capacities over the years.