If the level of math in ap physics C is basic calculus , then why do so many people think it’s so hard?
Perhaps because many people think basic calculus is hard 
In all seriousness, I’m not sure that it’s really all that hard (although E&M was the hardest AP for me). I think it’s because AP Physics C is one of the few AP’s that most closely mirrors a college class. i.e. Mech + E&M in one year is the same length that it would be in college, so it’s a lot of material to jam into one year.