Because difficulty is subjective. Some people might have the right ninjas and it's easy, for others it might not be.

And tbh, the thing isn't as hard as some make it out to be, it's reasonable if you do it in a team like you should. 5 star group instance was harder th*l the SA except maybe minato, and that's a toss up with one of the group instance (I think water main one?)

So instead of thinking "why is it harder and reward didn't increase", you should be thinking "oh, this is easy, good thing reward didn't decrease" on the easy ones. Expert mode aren't SUPPOSED to be easy.