We all know that demand creates supply. Students would not want to buy grades, no one would sell them. Everything is interconnected.
You are right, who wants to learn, he learns. Whoever spends time on other things, and waving his hand to study, he buys grades. And even if such a student gets a real diploma, this document does not mean anything.
There are also such paradoxes when a student wants to study, but he has no money. He studies everything on his own, then buys a diploma, as it is much cheaper, and gets a job.