Why is estimation part of refinement and not sprint planning?
Scrum teams plan continuously across multiple horizons. In the daily Scrum, the development team plans their daily activities and reassesses their ability to deliver the sprint goal. At the start of every sprint, the team needs to create a sprint backlog, giving them focus for a few weeks. Scrum teams also need long-term plans that help them understand when they are likely to meet business objectives or milestones; this is release planning.
Planning horizons
Each level of planning requires a different style of estimation. During the daily scrum, the development team will be creating a plan for the next 24 hours; this will require a certain amount of informal estimation to determine a reasonable amount of work for a day. During sprint planning, it is crucial to understand how much work can be completed within the sprint and therefore, how many product backlog items the team should select. Release planning takes a longer-term horizon and requires deeper estimates through the backlog so that release plans can be forecast.
Suppose estimation is limited to the sprint planning event and entirely focused on the product backlog items selected for the sprint. In that case, it will not be possible to estimate further down the product backlog providing the data needed for release planning. It may be possible to estimate further down the product backlog during sprint planning; however, this would reduce the focus on the purpose of the event and lower the team's ability to achieve its outcomes. The process of estimation often raises technical risks, challenges, or decisions. These can be disruptive to the sprint planning process as there is pressure to confirm the plan by the end of the timebox. If discovered during refinement, there is more time to decide how to resolve the issues discovered.
The scrum framework provides a mechanism whereby the development team can collaborate with the product owner to ensure the product catalog is in a ready state. Product backlog refinement happens outside of the prescribed scrum events and takes up no more than 10% of the development teams’ capability. During this time the development team will be able to spend their efforts working down the product backlog and providing the right amount of detail. Refinement includes the processes of estimating the product backlog sufficiently so that the product owner can forecast releases.