1. Most, but not all, of those large stones flushed out are formed overnight from bile and also from biliary sludge, often inside intestines. That is why they can be as large as feces.
There is plenty of evidence showing that. If you dye flush ingredients with a stable dye, you may find the dye inside some of the stones.
If you leave your stones out, in room temperature, they often melt into liquid bile.
2. Anyone can catch parasites, not just unhealthy people.
parasites have co-existed with humans for thousands of years, they evolved with humans, long before junk food was invented.