Is there any reasons why assertXXX methods in TestCase are instance methods and not staticmethods/classmethods?

Since they (to my knowledge) don't need to access an instance dict, they could be turned into instance-less methods, and thus be usable from other testing frameworks (like pytest, for those who want to use all the power of fixtures and yet benefit from advanced assertions, like Django's TestCase's assertXXX).

Am I missing something here?