Nobody in the league is willing to pay Marner what he thinks he's worth, and that is Auston Matthews money. Just anecdotally, I think it would make sense for any team to risk 4 first rounders and a massive contract to steal Auston Matthews away, but they wouldn't commit the same resources to acquiring Mitch Marner.
Now you can make the argument that Matthews vs Marner could play out like Lecavalier vs St. Louis where the small winger proves he's better than the big franchise center, in which case this thinking is wrong, but St. Louis' level of achievement was also several orders of magnitude higher than Marner's, over a much longer period of time, to be worth that investment.