In all seriousness, it's common, but as much as we like to tar and feather the kids we catch (some we even lay traps for), we rarely ask why and where it comes from.
Like adults, kids get busy. They may not have a mortgage or a car note, but they do have 6+ classes of work and tests, extracurriculars, siblings, home chores, jobs, etc. Throw hormonal changes, relationship drama, and how all of the aforementioned worries impact their emotions into the mix and you have a kid who is ready to explode. How do some students deal with it? Asking to copy a homework assignment, getting the answers to a quiz, or working cooperatively on a take home essay. Why? To relieve a little pressure. The grade focused world we live his only makes it worse.
The other reason is a lack of self confidence. Some students have been cheating off of the same kids since they were in 1st grade. Year after year it's reinforced,sometimes explicitly by teachers, that they are not smart enough to pass and need to cheat to succeed. If you don't believe you're capable of success without cheating or if you don't trust your brain to the point that literally anyone else's will do better than yours, you'll cheat. Some kids cheat because they literally don't know how to do anything else.
The sad/funny thing is that some of these kids cheat so well they can do it while you're sitting next to them handing them a completely different test. Remarkable. If our students put as much time into their studies as they do coming up with new and inventive ways of cheating, they'd have no reason to cheat in the first place.
So what are you saying? Don't punish cheating? Nope, punish it. But don't let that be the end of the situation.
Solutions? Build a relationship! Change the culture! All of those things people say, especially when they've never been in a classroom. Grades and tests are here to stay and so is the pressure they bring; however,explaining the role of these things has had some success for me and my classroom. Something that is there to help them by helping me know what the do and do not understand. Accurate information is key and is more important than the grade. Offering ample extra credit helps them believe me.