Because, as is, the Druid is a ridiculously powerful class already. Or, at least, I would think that was the rationale. First, they're full casters with a strong spell-list, only remotely challenged by the Wizard spell list. They have access to medium armor and shields, the best saving throw proficiency in the game that they can use to its fullest, good cantrips, and an ever evolving wild shape ability that provides wide ranging utility throughout all pillars of play. And that is all before any specific sub-class they later add grants other features to class. In the end, one has to make the determination that enough is enough.
