Why deflect melee cannot be used by mages, healers, Archers, daggers.

You have answer by yourself...
MEELE is close combat.

Most ranged units can't counter nearby attackers making squishy targets weird tanks in a pinch.

But the same can be said about melee units with deflect magic/range.

The issue is most mages are out of control with their glass cannon builds, this is Intelligent system trying to patch out the imbalances.

The reason is because IS says so. That's all there is to it.

Frankly, for game balance purposes. Much like when Pokemon introduced the Fairy type, which randomly hard countered the previously ubiquitous Dragon type.

The deflect seals were introduced to mitigate the power mages and brave archers had in the so-called "ORKO Meta" we were in recently (and are only partly out of). Allowing ranged units to run deflect melee would be counterproductive, as everyone would just slap it on their Rein/Lyn to make killing them with 1 attacker harder, and WoM3 dancers partnered with them even MORE efficient.

There's a reason why mages haven't really received any new toys post-Shadows of Valentia banners, sans the odd defensive additions to their arsenal, like Deirdre, Arvis, and more Ploy skills (which technically isn't mage-specific, just high res units).

That does raise an interesting point: there are no naturally close-countering tomes, only dragons can do that and it's technically not counted as magic. Now that would be interesting...

Ranged units just have less defensive options in general. You can't run any damage reduction specials on them either.