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For a similar reason that most coaches don’t actually participate with the athletes they’re training: their job is to watch the athletes, look for gaps in their technique, and help them to address those. You can’t look for that as effectively if you’re swimming alongside them compared to standing on the side of the pool.
But yeah, all “learn HOW to swim” lessons I’ve ever seen have the instructor in the water with the student, because they have to be like 25% teacher 75% lifeguard.