Because we play to his strengths - pressing forever and running into channels/dead ends all the time.While Doucouré is far from the most pleasing footballer to watch, we are arguably a far better team with him in the XI than when he isn't there.
He comes across as gangly and clumsy, but he's effective: be that pressing, getting the ball moving up the pitch or being in the right place to score.
If we need someone who can control the ball and lay a pass forward he's absolutely useless.
A clown who somehow hits a fair few in a team that needs those fair few is a net positive but he's an atrocious footballer.