Considering how many individuals are searching for them and want to claim them for their faction they are actually quite rare. It's not like they are going to fall into the adventurers' laps, they'll have to fight and/or bargain for them, maybe even steal them and run for their lives. Just because an adventure site contains an artifact it doesn't automagically mean that it ends up in the adventurers' hands. Plenty of other possible outcomes.How often are people giving out artifacts? The guidelines suggest that artifacts are a big deal and should be given out sparsely but there are a lot of artifacts in the book, which need to be put in front of players, and most adventure sites have the possibility of finding artifacts, so they are not actually that rare?