A lot of collectables are, to a certain extent, all or in part an investment medium, to the extent that their value is supported by active markets, established dealers, an interesting and recorded history, and detailed catalogues being available.
An example might be the philatelic market, where there are detailed catalogues for every country, and the opportunties for new collections and research. Hence many stamps are of great monetary "value". Arguable matchbox or cheese labels could be considered as attractive as postage stamps, but, while there are collectors and dealers, there isn't the same interest and, SFAIK, not the same availability of catalogues, magazines, etc., as philately. Hence not a regular market, and, again SFAIK, they are not of the same value as equally "rare" stamps.