Because, as defenders, sometimes you find yourself in sub-optimal situations and you're tasked with dealing with them.
Maguire did everything right, he waited for Dalot to get back in position to cover Welbeck and then Maguire tracked Milner's run. He did exactly what he's supposed to do.
Except, he wasn't fast enough to keep up with a 65 year old James Milner. The likes of Gabriel, Saliba, Konate, VVD are not allowing Milner to shoot the ball past our goalkeeper in that situation. None of these centre backs will be watching James Milner score past our keeper and saying "ah, there's nothing I could do because Welbeck is also on the field". They'd have just tackled him or blocked the shot because they don't run at the same speed as a tortoise.