I could never understand how the best football (soccer) players are allowed to participate in these things called friendlies. Aren't the teams and owners afraid of losing their investments of 10's of millions of dollars (euros) to injury? And it appears that teams just make up their own schedules.
In American football, there is no possible way a team would allow an extra game. Even in what we call the pre-season in our various sports, there is a limited, pre-determined schedule and that is it. That applies to all of the major sports.
Even in the games, the better players will be playing very limited time if at all. Even most of the NBA (basketball) owners don't really want their good players playing in the Olympics or anything where injury is possible. Baseball allows the pitchers only a couple of innings in a 9 inning games and they are out.
Pre-season games here are used primarily to look at new players and then give veterans some time to "warm up" prior to the actual season beginning.
I just never really understood why international football takes the risk.
Rub a Dub