Why would 560SEC Seat leather wear out?

Hi,
Just saw a 1990 560SEC with only 82K miles. Wood is faded and cracked, indicating suboptimal care.
More concerning to me is the complete wear through of the door side bolster of the driver seat.
Any ideas why/ when this occurs? I can't imagine the leather on an MB seat to completely wear though (big hole -exposing fabric below) under normal circumstances.
How much might it cost to repair this or would one replace the seat altoghether?
Thanks in advance for any input
farrukh