Firstly its true that the 100metre's is while hard to win, takes a lot less training than other sports and physically is a lot easier.
As others have said about the 200 and more the 400/800m you're pushing the physical limits. There are lots of reasons people slow in the final 10-20 of a 100m race, and its usually because the race is won in the first 60m during the acceleration rather than the rest of the race. Humans top speeds aren't massively different, acceleration is technique and power and has a much bigger difference in the 100m than cruising along at top speed, it also takes less power to maintain a speed than increase it, there is a point where the power put into trying to accelerate is worthless because you just can't go faster and a smoother "cruise" technique is faster than trying to power on. While its not absolutely relevant, how many people here can RUN 10km today, go out and just do it, how many people can run 100m, no matter how slowly? Its phsyically a FAR easier event, I could run 100m if absolutely forced to and I've got terrible joints, I literally couldn't run 400m, or 800m. Athlete's are the same, the level of lactic acid build up over a 800m race vs 100m means lactic acid barely effects a 100m race, in an 800m race you are literally racing through pain to reach the end, and training is the same thing.
Did you see the guy after the triathlon, the rowing 2km, the anything from 400m to marathon runs and the pain/exhaustion they are in, several being seen to be medical personel, carried to get help. When was the last time you saw a 100metre runner have problems like that? I've always disliked the fact that the 100m gets sooo much more attention than the vast majority of other events as its the least skilled, least difficult, least hard to train for and almost purely down to being born fast enough.
Also the heptathlon has less difference in sports than a triathlon/pentathlon, utterly stupid. Swimming, running, riding, all endurance sports, the athlete's train for endurance, that's it. Heptathlon, 100metre hurdles is POWER and technique, 800m is more endurance but still MUCH faster, the throwing events need utterly and completely different types of training, technique, strength.
You go for a long distance run and you're training for the swimming, running and cycling of a triathlon, because its endurance ability you're training and the only thing you need. Each event in the heptathlon needs entirely and separate training.