In short...
Demand significantly exceeds supply.
Particularly in the context of set-building with rare cards.
For example, although T206 Wagner, Plank and Magie are generally not "rare" from a supply-side perspective as compared to other contemporaneous sets, there are dozens, if not hundreds, of collectors who are working on or have largely completed a T206 set. These scarcities compared to the overall set make them frequently referred to as "rare".
Just my $0.02 -- this topic gets debated here frequently.
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