Decathletes / Heptathletes?

[Silva]

You are assuming that all multi-event sports are built by adding on an existing event with the same objective in mind, which they aren't. Decathlon discovers the most complete male athlete in track and field and Heptathlon the female. As the legend goes, Pentathlon would discover the athlete most suitable to "escape from being behind the enemy lines", Biathlon was about "border patrolling" and the Triathlon about finding the "fittest man". And if you look closely, you'll notice that each of those sports that combine radically different sports has their own federation: the ITU for Triathlon, Duathlon and Aquathlon (not the FIAA, UCI or the FINA) , IBU for Biathlon (not the FIS) and the UIPM for Pentathlon. As Decathlon and Heptathlon features exclusively events under the wing of the FIAA, they can organize the whole thing themselves, in stadiums pre-approved for athletics competition.

And more: Decathlon and Heptathlon can be disputed on any stadium with track and field facilities, where the other athletic events take part. By adding swimming, horseback riding, cycling, shooting, tennis or kayak you no longer can do that, as a lot of cities might not have close or even suitable locations for such sports, and the IAAF can't really demand "an olympic regulation sized pool" for a city hosting the World Championships and the number of cities capable of organizing events of the world circuit would get a lot shorter.

I really can't break this into more simple parts.