Because typically you do not estimate a story down at the task level until you are in sprint planning. If you are estimating stories at that level of detail as soon as it is created I imagine you have a very shallow backlog (doesn't go far into future). If you are creating a story on the spot, put in a rough estimate as a placeholder and then when you break down into sub-tasks you can remove the estimate from the story. That being said, I recommend leaving the "original estimate" as the sum of the tasks and the remaining estimate (still at the story level) as 0 so you can get valid metrics and forecasting.
Essentially, without an estimate in your story you have bad metrics. There is the workaround I mentioned above, though.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.