Swim sets but LSD runs - very different approaches. Why?

Most due to the technique aspect, yes. Technique is so crucial for the beginner that you should focus on it even at the complete cost of the fitness component. (Once you're an intermediate swimmer like sub 1:50-2:00 pace though the fitness component needs to also be worked.)

Another big factor is the logistics of doing sets in a pool and keeping track of pace. It's much easier to keep track of 100s,200s than it is for 1000s+. Add in the fact that if you're in a masters swim with 5 people in a 25yd lane, doing a 1000 straight would mean that swimmers would often be overtaking each other in the lane due to the length of the set, whereas this is much reduced with short sets.

That said, even though I'm not a great swimmer, I personally have still found benefit from doing the occasional long steady swim, like 1000+. As long as you pay strict attention to technique and pacing, you won't reinforce any bad habits, and you'll get real endurance training effects on the muscles/cardio.

Running is also a bit different than cycling/swimming in that your cerebellum (brainstem) is very good at automatically finding your optimal run cadence and run technique. You don't need to do special drills to pretty much run your optimal endurance form. This is clearly not the case for swimming where humans weren't specifically evolved to do, so you need to actively train it rather than passively train it. IN running you develop a great strong, smooth stride by running more and faster, not by just doing technique drills. 

Not only the relative (some would say paramount, at least for beginners) importance of technique in swimming and lack thereof in running, but the different nature of the two activities. Swimming is very low-impact (no impact?) and so easy on joints, cartilage, etc. If you have decent technique, warm up properly, and vary your strokes, it's possible to push hard almost every day in the pool and not get injured. Not that most triathletes would, or should, do that regularly, but competitive swimmers do.

With running, which is very high-impact, even elite athletes can't do high volume and intensity day after day without getting injured. I'm saying this in comparison to swimming. It's common for HS or age-group swimmers to do 10,000m a day of swimming, much of it speedwork; the equivalent in time/effort would be running about 40 km (25 miles) a day; very few runners can do this kind of mileage, and certainly not with most of it at high intensity. People vary in their tolerance for mileage and speed, but in general, it takes a lot of slower, steady running to build the endurance and strength needed to handle run speedwork.

The technique aspect is certainly true but there is another reason, too.  Swim training programs grow out of club and high school swim programs. Most swimming events in swim meets are short distance, 50, 100, 200, maybe a 400, with each swimmer in multiple heats and events over a day.  The training for those is based on shorter intervals for speed and for recovery.  People continue to fall back on those types of training regimens even if a longer intervals and continuous swims might be more appropriate for triathlon, particularly for long swim events like HIM and IM.

I personally swim a long, continous, slow swim just like running, usually 50 min to an hour.  I sometmes vary the strokes to rest but I don't stop.  Rarely do I swim intervals, like not in more than 5 years.  I  am not fast now and I am not really trying to get faster. I am trying to get full-body training with running, biking, swimming, and a little weight lifting. I want to avoid injury more than I want to get faster.  It works for me.  It won't get me on the podium.

TW