Why should lethality be melee only? It's simply another stat to scale champs by

League allows for players to be flexible in how they build their champions. Going full lethality takes up space that could be used for other scaling stats like crit, crit damage, attack speed, lifesteal etc.

When I first started playing league back in season 3 every adc had the exact same item build. Now there is legitimate diversity.

Caster ADC finally have a stat that works well with their play style and the trade off is comparatively less power outside of cooldowns.