SBR - Sale & Leaseback Question

I always find it difficult to explain why the ROU asset in a sale‑and‑leaseback is measured as a proportion of the asset’s previous carrying amount that relates to the rights retained. I understand the textbook definition, but I don’t understand the rationale behind it.

Why do we do this instead of simply recognising a profit or loss on disposal based on the sale proceeds minus carrying value, and then accounting for the lease in the usual way?