There are some classes where homework is more than 1/4 of the grade. Maybe not good math classes in the US.
Cheating doesn't happen when there's virtually no incentive. I stand by my point that you can cheat in Math homework more easily than equivalent non-Math homework, if you want to.
If 400-level Math coures are really impossible to cheat on, but there are 400-level non-Math courses that are possible to cheat on, I'd just argue that this is because they aren't really equivalent.
if your homework is 1/4 of your grade in the class, that class is a "pass me" class. in some math classes the prof refuses to acknowledge that homework actually has a measurable effect on your grade, but he will use its presence to round up instead of down (i.e. 89.5 becomes a 90 instead of an 89)