You can do that too as druid. At 20-30 you can solo 2-3 mobs easy, while hots are healing you up at the same time. If things go wrong for you, you reapply heals and hots + turn into bear and finish whatever is left. On top of that there’s far less micromanagement if you are a druid. So yeah… you might possibly do better as warlock but for twice the effort.
Don’t forget that 60-80% of your time spend while leveling is walking from place to place and here feral druid has big advantage at 20-40.