What's the reasoning for melee jobs to have three different sets of gear?

[Kazrah]

Samurai tends to wear heavier armor and such that it'd work just fine with Dragoon.

Have you seen the AF gear? Neither one of those sets are heavy armor.

Also have to bear in mind that (at least according to Yoshi-P) the inspiration for Samurai for FFXIV was the Edo period, post-military samurai that served less as an armored soldier and more as a diplomat and local security that also served as the same inspiration behind the Jedi in Star Wars.

Now realistically, I'd argue it would make more sense to make Ninja instead wear Aiming armor instead of trying to combine them with other melee.