Johnny runs to a loose ball to beat the opponent. But Johnny and the opponent collide and Johnny goes down hard. With such an obvious foul, why didn’t the ref whistle?
Just because a player falls doesn’t mean a foul occurred.
Sometimes two players collide equally (known as “50/50”), so neither committed a foul. If Johnny is smaller or was off-balance, he may go down, but that’s no reason to penalize the bigger or balanced player.
The ref makes a call upon seeing one player actually fouling an opponent, by pushing, holding, kicking, tripping, etc. If the ref doesn’t see such a foul, the ref can’t make a call, even if a player falls.
Note: A foul might have occurred, but refs can only call what they see. Sometimes a spectator had a better view — good for them! But refs can only use their own eyes, not those of spectators too.