That's true, but it's an artificial constraint done because of commercial and marketing
purposes, and does not have anything to do with photography and photography as art
as such. In my eyes an image isn't more "artsy" because it's a limited edition, but
I know some people see it differently thou.
I agree with Ronny. In fact numbering is a fairly recent development. Ansel Adams, Edward Weston, etc. did not number prints and only limited the edition number of portfolios. Adams actually said (I paraphrase) "why limit the number of prints one can make from a medium that is by nature unlimited and in which each print of an image is potentially as good as all other prints?"
I would add that by limiting an edition, one limits how much better later prints of a given image can be made when significant changes to the technology take place (as they do now)! What if you sell out the edition of a specific image, and then find out you could now print it better than you ever did before? If you number, and the edition is sold out, you are out of luck! You can't print this image anymore without breaking the promise you made to the collectors who purchased your work.
To me it seems that numbering comes out of a static approach to photography, an approach in which the artist believes, explicitely or implicitely, that he/she has made the best possible print from a specific image and will never ever be able to do any better. This no longer holds true today in a world where technical advances are made if not daily or weekly then monthly and definitly yearly.
When I realized this about 2 years ago I decided to stop numbering my prints. I only number my portfolios, the way Adams did, because they are collections of prints and not single images. They represent a completed body of work at a specific date and time. I also limit, by nature, prints that are unique, for example if I do paint touch ups to a canvas print. These are usually just one of a kind and are indicated as such.