You dont see them because of how rare they are in the first place, just like you dont see many Lambo Countach or Diablos.
They lose value fairly quickly, are expensive to keep on the road, require a lot of repairs and a 10 year old bentley may cost only as much as a 1 year old S550 or 750i but won't have the tech, toys, safety, features or warranty of a much newer, nicer driving car.
Also being such a niche brand, they go many years between model updates. A 2009 Arnage looks pretty much like a 1998 Arnage so you wouldn't be able to tell driving down the road that the Arnage next to you is 11 years old.
In the world of cars, especially interior design, which these super luxo-cruisers are famed for, 10 years is a lifetime. The 1999 Arnage mught still be a $50k car, but the interior is missing just about everything you'd expect in a 50K car like nav, satellite radio, stability control, modern ergonomics etc. IMO old luxo cruisers are impressive in badge only, there's really no reason to buy one unless you have some nostaligic or badge obsession.