Question: why is desalinated water costs calculated based on as if the water was immediately thrown back into the ocean? Wouldn't it rather be recycled after once desalinated?
Because it's easier to desalinate salt water than sewage?
How would salt end up back in sewage?
Recycled water is usually considered non-potable, so while it can be reused it's usually not available for drinking water and needs separate pipes.
Though there are processes that turn it back into potable water, just reusing it even once in agriculture would bring huge cost savings compared to having to desalinate it again, which is why I wonder why there doesen't seem to be any studies of the realistic likely costs of desalinated water when accounting for reasonable reuse