A reclined seat will restrict the ability of people seated in the row behind you to leave their own seat. Additionally, under certain cirumstances, your seatbelt may not be as effective on a reclined seatback, and may even cause injury.
I'm sure that Cabin Crew deeply regret the significant personal imposition and inconvenience that this request places on passengers when landing, who may already have experienced similarly outrageous demands from the jumped-up waiters and waitresses that airlines seem to employ as Cabin Crew these days....
