This is just my theory, I've never thought Coleman was an effective captain but the thinking might be that Frank doesn't want to further unsettle the dressing room by displacing him from that role even though he's heinous now. Theres a level of politics and a rigid club hierarchy that exists. If he wasn't the captain and a devoted club servant he would of been replaced years ago.
It's not just that is it? Coleman, regardless of what a lot of people think, is still capable as an Everton starter. Relative to what he was is he worse? Yes. But this manager knows what resources he has in the position and can still see that Seamus is the best option at the moment.
I know people are anxious to see a new signing whomever that is. But FL is tasked with blending the defence to the rest of the team...as was RB and DF. None of them saw anything other than Coleman as their starter at RB.
Maybe trust that judgement?