Why Project Scope Statement not an input to Estimate Activities Resources ?

Great question. First of all, something obvious: PMBOK is a guide so when you create your own process you can include the project scope statement in your estimate activities resources or not. Second, what you stated, comes from ancient versions of the PMBOK. But when you read in the last version of PMBOK the reasons about what the project scope statement is included inside the estimate activity durations it could confuse to everyone. Now, my answer that in this case is based on my understanding and experience but any other can think totally different. When you estimate acitvity resources you have a resource calendar on hand. The resource calendar is created at enterprise level. You do not need the project scope statement. Duration is not in your focus so the only thing you are doing is assigned resources to your activities, nothing more than that. The only thing you need are the inputs stated inside the PMBOK. When you estimate durations there are assumptions and constraints that will limit your decision about your schedule duration. And those assumptions and constraints are usually stated inside the project scope statement. Here, you can argue that some constraints will impact your decision about assigning resources to activities and I could agree with you. But remember that the process to do all this stuff is up to you. When PMBOK was created people who wrote the process think that in this way is more clear an easy to understand. And I agree with them. Hope it helps.