Hardly anyone does extreme endurance activities every day. The oxidative stress, tissue damage, immunosuppression, and inflammation can be pretty severe but with adequate recovery protocols there are unlikely to be persistent health problems beyond a few days.

Long term high volume endurance training can also cause calcification effects in some older athletes.

https://www.outsideonline.com/health/training-performance/en...

I had a friend who was really in to endurance sports for a while. He gradually kept ratcheting up the distance and frequency. At the time, he talked about it as chasing a feeling. I saw him once after he'd run something like 35 miles in a day and he was really relieved that he had found his limit at that distance. He was worried that he was going to end up one of the ultramarathoners running 50-100 miles in a day.

So yeah, if he's at all representative, I don't think it's health that they're chasing. The most I've done is a couple of sprint tris and a few 50-mile-per-day bike trips. But even that for me wasn't about health; it was about challenge.

They are proably associated with fit people because unfit people cannot do them.

After some distance/time how fit you are doesn't matter anymore. Then it all becomes about food to prevent all kind of physical issues.