It's just an aspect of the game today that managers can't do everything. There was a time when they were also practically the head scout but their schedule got so busy it was a good idea to hire someone to that role.
This model has been used in the continent for a long time, in fact in many countries they have never had a British-style manager who has a dual role. Manager's job is to make the best of the team he is provided. US sports is the same, btw. You have a general manager who deals with team building and all that and a head coach who is in charge of game-related things. The idea is simple: you can change the coach but you still have a guy in charge of the larger picture. You don't scrap everything that happened in the past two or three years just because the new guy has a new idea on how to build the team.