Because managing an international side is completely different to running a club side .
As I said in the original post Pep needs his players with him all the time because so much of what his teams do is automatic . The player on the ball will know without looking where his team mates should be therefore cutting down decision making time . This is not feasible when you only have the players for a few days before a game or a few weeks before a major competition .
Those same players will then be going back to their parent clubs and quite possibly playing a totally different way.
We have never had a foreign manager who could get as much from the players as Southgate has and as previously mentioned in this thread Sven Goran Ericson had players equally as good as the present crop and achieved very little.