Because powerscalers want to have set a set-in-stone power level chart ala DBZ so that all battles can be conclusively solved. If Hulk has a power-level of 12000 and Wolverine only has a power-level of 10000, Hulk has to always win, so they can call any writer who has Wolvie best Hulk hacks who are trying to ruin comics.

You're correct that there has to be some sense of scope with how strong certain characters are (Yes, it IS bullshit that Harley Quinn can dodge Superman and hog-tie Wonder Woman simply because she's "crazy"), but a good writer can do cool stuff with obviously uneven "match-ups".