When a team plays like nearly sixty games all season long, adding friendlies to that doesn't make sense, it would be just added fatigue for the players, most of whom ended up playing all through last year and during the summer as well, so it is not going to be easy at all, would be quite tough for them to keep going. So resting them, sending them on vacation and tell them to relax, would be a lot better.
All clubs have health directors and health personnel’s that are looking at the health of every player and how well or good a player can be throughout the season checking their health conditions. Exercising which includes playing of matches will help improve health condition of players naturally. But overdoing it could also have its side effects. PSG can play more friendly matches that they need to test their players and how good they can be when the season begins. They don’t need to play in those players that don’t look fit to take more matches throughout the season and that will help them reduce fatigue in the team. They have a lot to learn before the new season begins, they’ve lost some players to transfer and will bring new ones to play so they should try all they can to understand the game pattern they’ll use for next season.