It is in context of things which are not rare. For example the number of blondes at a beach party in Italy, which has a large brunette poopulation.

Imagine an all-blonde beach party in Siciliy. Pretty rare right?

So is a beach party where everyone is tall, or short, or has the same name, or a beach party during which there's an earthquake, a meteor etc etc.

All would be rare events, but there are so many rare events that could possibly happen that is is not that rare to witness something rare, especially if we are looking for that.

Therefore witnessing a rare event may not mean much.

This is related to p-hacking https://en.m.wikipedia.org/wiki/Data_dredging