Separating them before removal adds considerable complexity. You'll be dealing with the torque converter, potentially needing special tools to separate it, and wrestling with a heavier, less maneuverable transmission. Removing them together allows for a more straightforward lift out, often using an engine hoist. The combined weight, while higher, is better distributed and easier to manage with the right equipment. The extra work of separating them after removal is generally not worth the minor weight saving.
