Why Does the ID Movement Claim a Creator is the Only Alternative?

The Intelligent Design movement claims that lifeforms are too complex to have been created by natural selection. I don't belive they have a valid argument, but if they believe there's unaccounted-for complexity, why don't they consider the possibility that it's the result of some other, as yet unknown, naturalistic mechanism? After all, our current knowledge is hardly complete and final. Why is a creator the only acceptable answer?