I don't know how else to explain it. Bagley CAN run a pick and roll with Fox and Haliburton all day long with no problem. The issue is that Holmes can't do ANYTHING else.
At this point in time, Bagley takes half his shots from the perimeter and very few plays are run for him. So the difficulty level on the offensive side of the ball does not compare between the two. So if you're going to argue effiency numbers, remember they aren't playing the same role or even the same position. Holmes is a better help defender and shot blocker, but Bagley bodies up bigs better. Rebounding is a push. Walton favors Holmes because he's his type of guy. A role player. So he's going to cater to him.
Look, I have my issues with Bagley, but he's not the worst player in history as people like to make it out. Holmes isn't one the best 5s in the league either and he's not some defensive stud. The whole team stinks at defense. Barnes production drops off significantly when playing the 4 and he's not as good a rebounder as Holmes and Bagley. There is give and take with all of these guys.
As far as other teams being low on Bagley, I'm not so sure they are that low on his talent level. They may know that he's asked for a trade and that has put the Kings in a weak position. Could be the injury history, could be the dad.