Probably some amount of survivor bias and acceptance of injury involved. There are still riders (who in the fitting profession are called macro absorbers) who can ride double centuries on a wildly wrong sized bike on an ill fitting saddle wearing jeans and not face a single issue. Then there are people who just loved cycling enough to endure the ensuing injuries (some of which might have ended up being permanent). And then there were people who just didn't cycle because for them it sucked.
Today there is more knowledge of fitting, better selection of saddles, cockpit components, frame sizes and various geometries. All of these advancements have opened the door to people who back in the day would have found cycling intolerable.
My brother is someone who can ride anything with any gear imaginable. He did the full Iron Man riding the drops of his road bike because he couldn't be bothered to learn how to use an aerobar. I need to set my saddle to the quarter of a centimeter and tenth of a degree or it won't work. Luckily I can ride pretty much any chamois but bike fit for me is fiddly.
He would have been b cyclist 50 years ago. I would not have been.
And before anyone gets funny ideas about 'weakness' or 'lack of resolve', Eddy Merckx was notoriously fiddly with his fit and would adjust his seat height during a race whilst riding. And we all know he was not a weak rider. But he wasn't a macro absorber. He was a micro adjuster