Because the reality is humans don't really "know" anything the absolute certainty, but there are still ways of assessing which beliefs are more likely to be true than others.
There is always the possibility the findings of a peer reviewed, journal published, replicated study are wrong. They are just more likely to be correct than assertions without such support.
There is always the possibility the findings of a peer reviewed, journal published, replicated study are wrong. They are just more likely to be correct than assertions without such support.