What academic offerings can you find at Harvard that are NOT offered at UMich? That would seem to be the question that Harvard AO’s would be asking themselves. To quote Yale Transfer Admissions:
The same would apply to Harvard!
So, it’s not enough of a reason to say “I don’t feel academically challenged at UMich” because you could apply to many other colleges that might provide you with more rigor than what you could find at UMich.
So, what specifically does Harvard offer you that could be experienced NOWHERE else? (And your reason cannot have anything to do with prestige.) For example: Yale or Stanford might offer you more of a challenge than UMich, but what does Harvard offer you that you cannot find at Yale, Stanford or UMich?
If you can answer that question, it will help you be one of the successful 12 transfer admits that Harvard accepts from a pool of over 1500 transfer applicants.