Per minute is more precise and more useful - how do we know that? Because every football stat site out there has them, it just confuses the general public so they use games because even morons can understand that.Like you say, per minute system seems more precise, but you can’t really extrapolate from that; footballers are not some sort of algorithms who will put the ball in the net every 155 minutes or whatever regardless of circumstance.
Of course there are far more factors that go into a player assessment and of course you can't say Player X will score every 120 mins, same as you can't say he'll score every 1.5 games, but over a season per min it is a far more valuable tool than games per goal.