Yup or your design is just large. Due to the non-deterministic nature of the compiler guiding it at a high level makes it less likely to choose resources in weird locations. e.g. I roughly map out block ram assignments for some of the top level modules but still give it plenty of wiggle space.
Not only that but you don’t want to have to redo analysis and verification on blocks that have been already mapped, placed and routed. Especially as one does minor bug fixes towards the end of a design. It’s like refusing to use libraries.
