Why are training swims so long??

A typical swim workout is in the 45 - 60 minute range.  It allows you to get in a warm up, some main set work that may be focusing on endurance or speed or stroke, and then a cool down.

Because swimming is "no impact" you are able to get more work down with less stress on the joints.

The best way to know that you can "do the distance" is to have over-distance training under your belt.  But with the cost of recovery for longer runs and even some rides, it is not always possible.  With that said though, I will almost always have ridden close a 100 or so a few times before a HIM and will have run almost if not more than 13 in my typical long runs.  It all depends on your abilities and background/base.

Agree with everyone so far, but had to add this.

When I was looking for a new HIM training plan, I glanced at the 16-week Intermediate plan on this site (silver level) and saw that on Day 1, it had a 4750yd swim. That just seemed silly to me.

I rarely swim less than 2000 yards (about 40 minutes), so I either skip the swim workout altogether or stretch it to 2000 (sometimes longer). 

Impact and recover are part of the equation, but so is the fact that the swim portion of the race is so much shorter that it's hard to train and not exceed the race distance. 

Swim distances in the various training plans do not scale directly with race distance.  You'll probably find that OLY plans are also in the 3000 yard range for their longest swims even though the race is 1500 meters.  Similarly, my IM training plan calls for my longest swim to be 4500 yards (107% of race distance).  I do have considerably more 3000+ yard swims planned than I did for my HIM.   

I only train for sprints (and Olys this year) and always swim at least 2500 yards in practice - I've done as many as 3200 in one session. Sure, you could go and swim the race distance just straight each time you get in the pool, and yeah, you'll get better eventually. But doing sets and drills, etc, is more effective, I feel. Last night at tri team practice, our swim workout had us do a 1000 straight swim. Then a bunch of speed sets. So we all know we can swim 1000 straight. Some days, we do longer sets, some days we do lots of shorter sets, of varying degrees of exertion.  

Looking at your planned training you have one of the 3200 yard swim workouts on May 3, but this is also  a 1 hour swim session so the average is 1:52minutes/100 yards.   If you can swim at this speed then it's only a 1 hour session but if you are slower it will be longer.  I think you need to look at the goal of the particular swim workout and decide if you will do the total yardage or stop at the 1 hour mark.  For most of us I think we like to finish the distance even if we go over the time allotted in the plan.

I have quite a few friends training for their first HIM using one of the two BT plans. Both seems to have huge swim volume and the beginner one is relatively low on run/bike volume especially if you are a women or slower person.

I have suggested to others to do the swims from the beginner HIM plan and the run/bikes from the other plan.

I'm training for HIM in 14 weeks and IM about 10 weeks later and those doing same race and following BT plan are swimming longer than I am currently.

Accdg to your logs, you can get about 2K in 45 mintues or so, we're only talking another 20-25 or so minutes.

I am a FIRM believer at the HIM/IM distance to train the swim HARD.  Yes, it is a smaller percentage of the race but (1) as others have said, you can go overdistance without the same strains as bike and esp. run and (2) there is a huge difference in the rest of your day between "surviving" the swim and getting through the swim feeling as if it's an easy warmup.

Yea, I started to go with that plan last year.  Realized it was a bit more than my definition of intermediate.

The Swim is so short it makes it worthwhile to go long.  If you double a 2000 you can still get it in 1hr 30min.  That's probably faster than your 13 mile time and maybe an 1hr 30min longer than your bike.  So instead of doubling your swim my advice is doubling your bike.

Many athletes failt to realize the benefits you can get from swimming more. We are not talking about crazy swim improvements; yes usually by swimming more you get a bit faster in the water but the aerobic benefit we get from swimming helps us improve our bike and run times: 1) it allows us to swim either faster or easier conserving energy 2) low impact exercise; it allows us to get 1+ hr doses of exercise with relatively low fatigue impact 3) upper body resistance training.