Imagine that you are Gretchen Whitmer and you are thinking about mounting a campaign for the nomination. Since the delegates were originally pledged to Biden and he has endorsed Harris, your chances of success are slim. If you fail, you will be roundly attacked for being divisive and damaging the party in the existential battle against Trump, which will hurt your chances in 2028. If, against the odds, you succeed, you will have to put together a national campaign in record time even though you have no experience running outside of Michigan. You will have to do something to deal with the anger of the Harris supporters, who will feel their candidate is entitled to the nomination. In addition, you would be starting the campaign as a big underdog, given the current state of the polls.
Is it any wonder she is not, in fact, mounting a campaign for the nomination? Running against a highly unpopular J.D. Vance in 2028 sounds like a much better bet.