I think something key to understanding that question is taking into account the idea of class fantasy in designing classes. Sure there are aspects of druids that do come off cleric-y (though it's worth remembering that it's assumed all people in the setting worship a god, so of course someone devoted to nature would also worship the nature god, same as Gale loving magic and worshipping the magic god) but the fantasy of being a cleric and being a druid are different things. People play a druid because they want to play someone who's first and foremost in tune with nature. That could certainly come with deep devotion to a nature deity but that's secondary to being devoted to NATURE itself, something that isn't displayed so much in the druids we see in BG3. Someone plays a cleric because they want to be someone for whom their relationship with their god is central. I think saying druids should be folded into clerics would be like saying paladins should be folded into clerics or fighters. Yeah you could do that and logically it wouldn't be crazy. After all, wouldn't it just be the equivalentof an arcane warrior fighter but with divine spells? Or a cleric with some extra fighter flavor? But they're all different classes because they fulfil different fantasies in terms of roleplay (and while we're on the subject, I would argue that Arcane Warrior should be its own subclass instead of just a subclass of Fighter as well).
