I think we Humans are kind of Jack of all trades, maybe Humans shouldn't be allowed to be Druid, but the only reason I can think is for races balance.
There's a lack of lore to explain why humans can't be Druids or any other class, I mean they were friends of Bearan for so long, even their will for life and our sentimental nature can explain why humans can be Necromancers.
1st edit: After reading some comments I realized I forgot about the humans-undead lore, so humans shouldn't be able, only the villains like Joul Candio and Leyen said but that's kind of contradictory because you stated in race description that humans can use magic from different sources so ...
I agree. I would say most fitting to least fitting human classes would look something like:
Warrior, Ranger, Assassin, Pirate
Priest, Paladin, Mage
Witch, Necromancer, Druid
Top line are must haves, embody humans perfectly, should be 'competitively the best' at those roles.
Middle line are classes I think human should be but maybe middle of the road for those roles.
Bottom line are classes human could be but should be the least proficient in if so (compared to other races).