What makes you say this? Is it based on personal experience, definitions, or some consensus? I have definitely known people who found things interesting but still thought they were hard. I have friends who find math fun and engaging, but also find it hard and (seemingly rightfully so) don't think they're very good at it.

Yes, personal experience and watching other people around me as well. Mostly, hard is boring, i.e. when you don't have enough interest to pursue it. As for math - it is not just about being good, but also the process itself - of becoming good, i.e. learning. When this process seems interesting and fun, I doubt someone will say it is hard.

E.g. for many people, literature classes or some other humanities stuff is hard.

>when this process seems interesting and fun, I doubt someone will say it is hard

That doesn't mesh with my personal experience. I know some very, very smart people, who have found the process itself interesting and fun, but who still find it hard and also don't think they're that good at it. Perhaps your personal experience is difference, but I've seen that interesting and fun does not preclude subjective feelings of hardness.