Why swim so much in custom plans?

It's true.  These plans do have some decent swim volume but it's from the fact that so many beginners are actually underprepared in the swim - we want to make sure you have no problems which is why you see the overdistance training.  Plus since swimming offers 'no pounding' of the joints, the extra training is great for your aerobic system without the risk of injury which will translate in the bike and run.  Instead of out of breath, HR through the roof and wiped out coming into T1 for a poor beginning bike performance, you will be a lot better prepared to get on your bike and go.

I suspect you will get some blowback from the group that says swim training will NOT translate on the bike and run.  As an example of a highly trained sprint distance racer, who is a threat to overall podium any AG sprint race at any level, my kid's TP account shows that 53% of his yearly training volume (by duration) is swimming.  Yes, he will log nearly 1.5 million yards, but his bike training will only be about 1200 miles and his run training will end up totaling about 700 miles.  He stays remarkably injury free which we attribute largely to low run miles and big swim work.  Also, likely because of his age, he is able to do very intense run and bike workouts at a much greater frequency than most of us may be able to handle, but the intensity can also be relative to your age and fitness level and you may still see great improvements.  To me, the problewm with most triathletes is that they are completely unwilling to put in the type of swim work that can yield great aerobic fitness gains.....or they have no idea how.

It's funny in a way.....you can now look at any major race like the Chicago ITU sprint race and see the top 20 places overall dominated by young kids who are straight out of swim programs with very little comparative bike and run training.  Coincidence?

I have had private coaches that I worked with 8 years and I have never had more minutes swimming in a month than bike or running. I don't think I'd follow a plan like that. For many a sport 3-4x a week is pretty normal.

Can you share what inputs you put in and in general the output and perhaps others can give you some advice?  Key would be to know what distance you are training for and how many total hours you can train.