Because progressive overload and eating enough will get you bigger.
Say you train exclusively in the 3-5 rep range, going from a 315 5RM to a 405 5RM and getting stronger. Even having not trained in the 10-12 rep range at all you have in fact gotten stronger in the 10-12 rep range, having increased your 12 RM and being able to do more reps at a given weight if you tested it.
It’s efficient to train in the 3-5 rep range to gain muscle. High reps might be slightly more efficient and have other pros/cons but within reason all rep ranges will get you gains.