
Several issues.
-Some students are inadequate in English language.
-Some could do the work, but it takes them much longer.
-Many of them take very heavy coursework loads.
-Many of them come from privileged backgrounds, and buying their way through feels natural.
-Many are majoring in math/science or business, so they aren't "language people" in the first place.
-The schools have not mastered ways to combat the types of cheating these students do.
-The schools make a lot of money from international students and are not too keen to upset the apple cart.
There's enough blame to go around for both the schools and the students. The essay industry is also an enabler. It's somewhat of a balanced ecosystem right now, since everyone is getting what they want.