Why do students cheat? Because they can. People, in general, always strive to avoid the extra effort that is inherent in us initially. To condemn it is absolutely pointless. If you write off - you save a lot of time and effort. Do not write off - your diploma still does not cost anything because of the total prevalence of cheating by others. It remains the only motivation not to write off, but to teach - if knowledge is provided in pairs, which in practice will be useful after graduation.
In writing, an important role is played by cultural differences, here I also refer to education. In post-Soviet countries pupils / students pay more attention to what they received, not what they learned. The main reason for the write-off is the lack of understanding of the result. On the other hand, you need to pay attention to the items that are written off. For example, in a situation with mathematics, chemistry or physics, writing can be explained by the complexity of the material.