I've said it multiple times, they dont design it because they don't care.
This series is meant to be played by kids to have fun, not for a challenge. It's meant to be easy.
Any time they actually bother to implement a challenge is to throw a bone to veterans, not something they are willing to actually invest resources for.
(Yes, probably any other company would have invested in making challenge modes for the sake of hitting a second target audience, but GF/TPCI are fine with just pleasing their target audience and getting anything else as collateral extra money)
The thing is: judging by how old the franchise is, they need to realise that a big portion of the fanbase starts to be OG. The game can be easy (default mode), but a little love for the OG fan portion could be present (a difficult mode). The target audience is new kids, and that's fine, but a big fraction of the fanbase is old enough to question things and deserve to be heard as well. But this is my personal opinion, it is how I see the franchise should evolve.