Seeing Red Again

[Genz]

Though it still doesn't answer why the melee skills are usable before charging mana because they're pointless without it.

Because FFXIV has a lot of small design errors and incoherences like these to look interesting. If they removed them, we'd have "less skills" and some players would complain, even if the gameplay would be exactly the same.

Combo in general are the best example : second and third skills of a combo are useless alone, but we can use them.
(when they added combo in 1.20, there were some case where using the second skill alone had a different effect)
But as Dragoon for example, we can't use Fang and Claw and Wheeling Thrust outside the combo. ¯\_(ツ)_/¯
And in PvP, the devs did have the realization to put the combos on 1 skill.
Also all the skills that get automatically replaced when some condition are met, like True Thrust -> Raiden Thrust : we could argue that it needs to be a separate skill and it's down to the player to have the "skill" to use it correctly.

tl;dr : it's just arbitrary.