I am not an expert for 5E.
What makes druids so good?
In BG2 Jaheira was good for insect plague to stop everyone from casting and panic some of them.
Apart from stone skin she had nothing tanky, as far as I remember.
In 3E they had an animal companion and they could turn every spell into a summon monster spell.
In those games I almost never used shapeshift, Jaheira as fighter/druid was better with weapon and armor and in NWN2 the druid companion was mostly a heal and buff bot because there was no cleric early. Her badger was useless so I made him a dinosaur.
As Vic pointed out, it is because of shapeshifting. The way it works is that your hit points are replaced by those of your wild shape.
And at level 20 you gain unlimited uses of shapeshift per day, and can transform into an earth elemental at will and gain 126 hit points and also take half damage from nonmagical attacks.
