if we start CB who is not good on balls, even with GK's awful distribution, we'd be ripping apart because they'll get more time on the ball, they'd be pressing us so hard due to us not good playing out from back, and the fact it helps them to attack smoothly when many of their players already were in our third final with De Gea gifting them easily possession due to his awful distribution.
Enough good to play out from the back to help midfield in possession and buildup so opponents won't have a lot of times on balls, we can keep them so far away from our goalpost so we don't have to do more defensive work if we can play out from the back. A team like West Ham, Everton, Watford, Southampton will never press our backline if they knew we can play out from the back because that would mean they'd be leaving massive space for us to bypass and exploit space, that's why they didn't do the same thing against City and Liverpool. We don't have enough players in backline who can play from back comfortably.
If we can't play out from the back, we'll have to do more defensive work rate because we can't keep possession. If we do more defensive work, more likely we'll make mistake in defensive. Less defensive work rate, less mistake we make. Defence and GK put more pressure on midfield than midfield put pressure on the defence.