Because if you play to a 1 - 2 - 3 - 4 pattern, and the 2 and 4 are the accented strong beats, you learn to swing. So we set the beat so it only plays 2 and 4. Think of how a jazz player counts the band in. He snaps his fingers on 2 and 4, never on 1 and 3. Like the videos posted above by https://www.strat-talk.com/members/dogletnoir.40400/.