Because most of our best players all essentially want to play in the 10 space and to play intricate football. That works at City as the whole team is set up for it and train for years to do it - most players come into the City squad and struggle for their first year while they learn the system. Southgate is not as good a tactical manager as Pep, and even if he was he can't get such an intricate system going to fit all our 10s in his limited time with the players.
Most good international sides have width, pace, midfielders who can take the ball under pressure, and a solid defence. We lack wingers and press resistant midfielders. And even those we have, Southgate doesn't play as to his mind Foden, Bellingham and Kane have to start even though they all take the same space.
And before you mention Spain in the 2010s, that was a bit of a freak event as it was basically a team from two clubs.