Here is one big reason why "cheating" happens. It isnt directly related to the high school level, and more applicable to the college, though.
The problem is that different classes/subjects/majors have wildly different expectations with regards to how much you are allowed to collaborate with other students.
For example, in my business and economics classes I took, collaboration is encouraged by the professors so much that everyone just works together on everything, without even asking if it is cheating, because of course it isn't.
And people who take these classes then go on to take a class in a different subject, where you are not in a situation where literally every single assignment is a group project, and they forget to ask if what they are doing is allowed.
The problem is that different classes/subjects/majors have wildly different expectations with regards to how much you are allowed to collaborate with other students.
For example, in my business and economics classes I took, collaboration is encouraged by the professors so much that everyone just works together on everything, without even asking if it is cheating, because of course it isn't.
And people who take these classes then go on to take a class in a different subject, where you are not in a situation where literally every single assignment is a group project, and they forget to ask if what they are doing is allowed.