Requirements phases miss edge cases.
Most software projects overrun because the requirements phase missed the edge cases. Everyone catalogs the happy path. Almost nobody catalogs the twelve failure modes, the seven regulatory constraints, the four adversarial inputs, and the state the system finds itself in when a background job fails midway through.
Those gaps get discovered mid-sprint, and every one of them is a decision that should have been made in week one — being made in week nine with a deadline breathing down the team's neck.