It's in part a cap space thing for me - Grant is a fit for a team who wants to contend, but lack of upside (and, i suspect, his preferences) mean that the Indiana/Houston types are less likely to sign him for that. Sacramento is a possibility and a fit---and, they have other things they might do. I suspect the different 'reads' are why he didn't sign last offseason---Celtics think the market is more like I am thinking (they were right about that when Smart was in similar situation as RFA as I recall) and Grant/agent sees the market more like you do. Utah and SA are both wildcards with him in my view too.
So, the set of conteders who want his profile is not huge. And there's likely-better players (Barnes, Draymond, Jerami Grant, arguably Lopez) chasing the contender dollars as primarily bigs. Plus guys like Middleton, Van Vleet, Kuzma who will chase dollars/slots for cap space teams. Someone might see him as a guy with upside too.
Now, the wildcard is how many of those guys resign where they are (and thus don't eat up space for Ind/Houston etc). And I realize there's lots of more creative ways for someone to add Grant...I guess I just suspect his market is more limited, and that doesn't exclude one team from really wanting him and paying up for it. We will see---I just think he's in an interesting spot, as are Celtics.
Personally, I'm happy to have him back though my ideal is to sign and trade him either for a pick or an interesting younger wing (Deandre Hunter is someone who I could possibly see, though lots of reasons that wouldn't fit in each direction and would guess Hawks wouldn't be willing to do that). I personally would not dump Brogdon to keep Grant, and doubt the team will.