The fact that humans can’t be a druid has always been funny to me, maybe for you as well. I know this isn’t D&D but come on now.
Most of my characters are Horde though so this doesn’t affect me, but for those who do play Alliance this would be a huge W.
The Horde are happy to fight for Alliance rights 
And there is the explanation.
Celestine of the Harvest used to be human and is a Druid before turning puppy mode.
But we have to ask ourselves why can’t it be so?
Kul’tiran moment 
IIRC, there was a (Knaack?) book character that was a human invited to the CC to learn how to druid. I don’t think he’s ever shown in WoW, though.
Maybe people don’t want to play an overweight druid that looks bad in everything 
Because this is the World of Warcraft, not the World of Greyhawk.
Blizzard’s excuse for stuff like that is usually well they are just a really skilled alchemist.
That’s the nonsense they try to pull with Blood Elf boss High Botanist Freywinn the Botanica dungeon, he is a Blood Elf wearing tier 1 druid armor and casts Tranquility and Tree Form while also summoning a bunch of little plant monsters. Dude looks like a druid, casts spells like a druid but seemingly isn’t one only because playable Blood Elves cannot be druids.
Th “explanation” is more like Blizzard doesnt/has not made the form needed for humans(or for the other non-druid capable races at this point) Blizzard has said they want all races/class combos be added someday and I think that will happen. Someday, but this is the sort of time frame as say waiting for everyone to get heritage armor.
Then don’t play a human Druid 
they have baller as hell Druid forms and it’s not like you’re outside of form terribly often (unless you’re Resto).
They also have really cool lore, big fan of Kul’tirans
Human Druids would look like naked mole rats.
Sure, if it helps you.
They can’t even grow beards and you expect them to turn into a bear?!?
They can be, they’re just called Kul’Tiran
It’s crazy that I can shape shift into a dolphin and a bird but not a human
Druids should not even have races, just let them shape shift into any one they want
I would choose night elf still, so hunk mode. Then gnome when I want to feel cute
You know that this will happen right? Especially considering we are now getting void elf demon hunters and soon probably paladin void elf/kul tirans.
The gate for what is “lore acceptable” as an excuse for not having a class was busted the moment we got warlock lightforged.
The fact the red manari skin for Draenei is usable by Paladins and Shamans is a big screw you to the lore too.
And yes Lightforged Draenei can indeed be Warlocks further muddying the waters
I’m sure you’ll get a lot of stuff about culture and ‘race’ and etc – when it comes to this question, people tend to conflate tendencies with absolutes.
They could just add an option of the regular human models as Kul Tiran.
This is already the case with most Kul Tiran NPCs; they use the standard Human model.
The reverse could also be done, with Kul Tiran models being made available to Stormwind humans. We see both models used for Arathi NPCs who are obviously neither Kul Tiran or from Stormwind.