Note I said "some amount of d6", with the exact amount of d6 left undefined. By having it not require an attack roll, bypass DR, and not have SR, you effectively make the minimum amount of d6 to obtain a usable spell smaller. It's guaranteed damage, thus fewer dice need to be rolled to satisfy people.
For example, I could make a spell that does 10d6, with a save for half. Or I could make a spell that does 5d6, no save. The latter is more conservative in that it does less damage, but more consistently. It also neatly avoids any twinking tricks that people might use to boost their save DCs into the stratosphere. The same principle applies with attack rolls, SR, and whatnot.