Why would he want to take over a club that's in the middle of its season, and a semi-severe performance crisis as well? No preparation time, immediate need to deliver in a very challenging league with a squad that is questionable to be of top-6 quality, no possibility to change the squad for a few more months... not exactly an attractive proposal. Especially not when you stack a shortterm contract on top of that.
And the other way around, I really don't think that Tuchel is a good fit for United. He craves exactly the comprehensive, unchecked power that the club is trying to scale back from and put onto multiple shoulders. He's not exactly an easy character to work with either, is my impression at at least.