That not entirely true - mages are almost as bad but the energy u need is less and there is always eboost if you wanna waste points.It is not even close and it is well known...
Druids energy costs are stupid high which makes them way under powered compared to all other classes.
Sigs don't work with bosses that spam aoe damage
Don't forget sigils don't work on lava or under pox effect or any poison in general in addition to aoe bosses.
Also even with sigils you need a huge amount as you can't take long breaks when healing a party the breaks need to be infrequent and short. Thus 500+ for 120 Druids to 2000+ when you are 190. And that's still not great regen believe it or not.