Cars rot away just as badly as any motorcycle, it is just that half of a cars bodywork is made up of plastic panels and rubber.

You can either buy a motorcycle and polish it every day and keep it in a shed, or you can ride it and enjoy it :P

At the end of the day a well ridden 2 year old serviced bike with 15000 miles on the clock will be worth nearly as much as a pampered polished machine with no service history.