This shows a quite worrying lack of logic.
First, you may have worked in "an environment" where people were high earners, but how many of those people did you personally interact with on a daily basis, and be close enough to be privy to their intimate medical problems? Every job has turnover, and most jobs have people who wfh or have flexible working -- unless you've personally kept track of every single person who's ever left your company (which would indicate a tiny company, see point 2) and you've also personally investigated every single co-worker who's ever done flexible working or had a period of leave (ditto) then you have no idea how many people left or went on flexible working due to chronic illness.
Second, what's the actual number of people in question, because you're extrapolating for the entire country based on what logically must be a tiny sample size?
Third, you're assuming that anyone with a chronic illness will be obviously visibly "clearly unfit" which is just bigotry. One of my close friends has a severe chronic illness and has had multiple hospitalisations and surgeries, yet she's a very successful actress and has starred in major theatre productions in the West End and starred on TV shows without anyone except a trusted view ever knowing she was ill. Plenty of people with chronic illness work, and no one ever knows.
Fourth, very few jobs pay £100k straight out of uni. You usually have to spend years working in positions that require very long hours, often very stressful environments, before you start earning that kind of money. Those jobs naturally draw people who have a much high level of stamina and capacity for stress than the average person. Obviously many people with chronic illness are simply not physically capable of putting in all years of working 100 hour weeks or whatever to get to the point where they're earning £100k, so it makes sense that your "environment" selects them out before they even get to that point.
It's a very weird point to make - a bit like saying "I'm an Olympic sprinter and funnily enough out of all the thousands of other sprinters I've competed with over the decades, I've never met even a single one who's a wheelchair user, HMMM how suspicious."
Fifth, confirmation bias is a very real thing. Plus it's just common sense that people tend to be drawn to those like them. You have judgemental attitudes about disability, so it's likely that disabled/chronically ill people in your "environment" (whether consciously or subconsciously) pick up on that and avoid getting close to you.