Because they consist of a set of EC2 instances, the vertical and horizontal scale of which is determined automatically based on the average traffic profile of each node. Once the traffic has increased enough to warrant a scaling event, it takes minutes for new ELB nodes to come online and go into DNS rotation before they can start serving traffic.

It's obnoxious that AWS hasn't developed an API call or web interface option for the ELBs to pre-warm them yourself, vs having to contact AWS support to get the pre-warming done "manually".