Because here, you can do all that for notably less money - we have something called Lincoln, which sells highway cruiser of the year Town Car (with a 4.6-liter V8) for $2k less than a base S-Type (with a 3.0L V6). Furthermore, I'm not sure if you're aware of this, but Lincoln also sells a version of the Jaguar S-Type here called the LS - base price of that is a hefty $12k less than a base S-Type, with the same engine (granted, the S-Type does have a shocking three horsepower gain).
Or you can move over to Buick and pick up a top-range Park Avenue Ultra, which is loaded with more features than any Jaguar without options could ever wish. This vehicle has a supercharged 3.8-liter V6 with more power than Jaguar's 3-liter, and is among the quieter vehicles I've ever ridden in. Base price? $4k less than a base S-Type.
Cadillac's another brand that'll give you a good deal on a highway cruiser. The feature-laden Deville starts at $2k more than an S-Type, but with 40 more horsepower and lots more standard features. Or you could go down a notch to the rear-drive STS and get basically the same car as a base S-Type except with 20 more horsepower and for $4k less.
Perhaps in the UK you guys think Jaguar is a good deal, but around here, Jaguar is for suckers. If you wanted a dynamically crappy highway car, domestic brands offer great solutions for good money. Meanwhile if you wanted dynamics and performance, other European brands are there to steal the show. Jaguar's in trouble. Case in point: the X-Type didn't even sell a third of what they thought it would, and this year they cancelled the model's base engine. Their sales prediction for the all-new 2005 X-Type wagon? 100. Per year. Judging by the fact that I was on a Jaguar lot last weekend and didn't see one (and it's been out since February and I still haven't seen one), I'd say that was a liberal estimate. In the US, Jaguar is a floundering brand.