I also think that Stevens is very loyal to Butler since he's from Indiana.
We have to consider that Stevens, who is seemingly universally loved minus IU fans, will not want to face a backlash for leaving the university (from his home state) for a school like Duke or UNC. He won't be seen as loyal for leaving the university that took a chance on him
It's sorta like the Lebron move but not exactly. Lebron made a circus of it, held an organization hostage, and was seen as engaging in collusion (for teaming up with other elite talent) but Lebron definitely left his home state + home team before fulfilling his promise of winning a Championship.
If Stevens win a Championship at Butler before Duke/UNC comes calling, then he has no reason to leave. He has already accomplished the ultimate goal. He can then set out to make Butler into a new powerhouse.