As an exI'm talking about nothing more than how the eye works and deals with 'flat distance'..... I know from playing in goal as a teenager it's not until play comes into your half that you can start to see verticality(*) or speed properly....
(*As in vertical passing lanes, not in a ground-to-sky sense.)
No doubt a pro develops a better feel for the game, but the same fundamental disadvantage applies.
Of course, that doesn't mean a keeper shoudn't be captain. After all Hugo and Zoff have done well as captain. There needs to be leaders all over the pitch imo. A prime example of that would be Utd of years past;
Schmeichel
Bruce
Pallister
Keane
Cantona
etc etc
All could of been captain with no bother at all.