Originally Posted by hhnngg1

Mainly because most triathletes suck at swimming so the gains are potentially big at first.

However, once you're even to middle-of-pack, gaining speed in the swim is the most work for least yield in triathlon. Cutting off 1 minute off your 28 minute Olympic swim might take 50% more swimming, whereas if you applied that same time to the bike, you'd gain 3-4 minutes. The exponential resistance of water means that going faster takes a LOT more energy and technique than it does for bike/run, so it's well known that once you're better than average, swim gains are extremely hard - harder than bike/run gains to the point that it's usually not worth pursuing those small gains if you haven't maxxed the run/bike. This is even for sprint/oly races where the swim isn't a small % of the race (like an IM).