This is a bit like saying that Magical DPS are homogeneous for them all having some degree of cast times on their spells. There comes a point where all jobs in a role category have to have some degree of common gameplay features, or else it ceases to be a category.
You don't necessarily need a lore justification for this. Does a painter have cast times when painting a picture? If so, why should Pictomancer have cast times? The reason is simple. If you take away the cast times, it ceases to belong within the role category. You've created a physical ranged job, but you've just mislabeled it as magical ranged.
Front positionals tend not to be a thing because then you have to design all your fights without cleaves, which makes tank gameplay less interesting (although a lot of fights opt for this role-based simplification). Samurai fought in wars, so I doubt they had a rule against refusing to kill an enemy soldier who had carelessly left their flank or rear unguarded in battle. Can you imagine how ineffective that would be in actual combat? You could just keep your back to them while attacking and they wouldn't be allowed to hit you.
With regards to your earlier comment, the reason why Physical Ranged don't have a role defining mechanic is because any time one is added, it gets phased out due to complaints. I think if you allow to happen on every role, all jobs will converge into that very design as an amorphous mess, and that's where the real homogenization lies.
You do realize that any existing complexity in RPR's rotation is due to the existence of Death's Design, right? It's the entire basis of Double/Triple Enshroud.