Short answer: they're not.
Long answer: This question actually comes up quite rarely, because the overall page loading and rendering time is so good that most people don't notice how large the resources they are downloading are.
But it's a question worth answering since it's interesting from a technical point of view.
The essential point is that over-compressed images full of artifacts look awful. As long as loading and rendering time is not negatively affected, higher quality images are always going to provide a better experience to our users.
Our pages are designed first and foremost for interactive use by human visitors and not for indexing by search engines. Search engines love it when you compress your images beyond the point of no return, because it's drastically reduces their bandwidth requirements. They have virtually no vested interest in the visual quality of your material, and once the majority of the web has been convinced to over-compress their images and destroy the fine details, then it's just a race to the bottom.
As far as we know, the vast majority of people visiting our site are doing so over modern fixed line broadband or cellular links. Certainly, you should be experiencing total page loading times of around one second when moving from page to page, and the time to initial rending should be below one second.
If the site is ever significantly slower than this, it's usually because we've taken one of our DNS servers off line and DNS requests are timing out. In other words, it's not a website design issue.
We are one of just a handful of websites globally who support arithmetic JPEG which reduces the image bandwidth requirement by around 10% for exactly the same quality. It's not enabled by default, since there is no reliable way to detect browser support, but it can be enabled on the website configuration page. Once again, we're ahead of the curve, doing something that virtually nobody else is. Please try it out before complaining.
Furthermore, despite clearly asking people not to infringe the copyright in our work, we've seen it plastered around the web everywhere from unauthorised, (and un-necessary), ‘archives’, of our material, to use of our images on message boards. Since most of this re-use involves re-compression of the material, and some people will inevitably get their first exposure to Exotic Silicon via these unauthorised copies, we at least want to minimise the damage by ensuring that the original material is of high quality. In this way, any ‘pirate’ copies that have been further compressed at least have a chance of retaining the high visual quality that we want associated with our original work.
By the way, have we mentioned that you can browse our website just fine with images completely disabled?