That with the pitch release countdown are supposed to improve the pace of the game and make baseball more "interesting." The six visits include catchers and coaches so if there's a man on second and the catcher wants to confirm signs so they don't get crossed up, too bad. Or if a starter is having a meltdown, there might no be anybody to go calm him down or they run out of mound visits for late in the game.
Why not play six innings? That would also improve how long it takes to play a ball game.