Because it means the offense went nowhere for 3 plays, which is the true meaning of a 3 and out. They only got a FG because they got the ball in FG range. So it was a big fat failure on the offense. They ran 3 plays and out goes the offense and in comes a kicking team.If you go 3 downs and score by kicking a FG, how is that a 3 and out? You scored.
3 and out means that you run 3 plays, you don't get the first down, you don't score anything, and YOU HAVE TO PUNT.