The word "archives" refers both to the places and the materials housed within them.
1. Archives as things
Archives are materials (reports, digital files, photographs, correspondence, social media posts, field notes, scrapbooks, etc.) that come about as a result of someone (a person, family, or organization) doing something AND that are determined to have long term historical and evidential value. (A more complete definition is available from the Society of American Archivists)
Enduring value + created during the conduct of affairs = archival materials
2. Archives as places
A physical place where archives are kept and/or an organizational unit responsible for taking care of archival materials. (A more complete definition is available from the Society of American Archivists)
1. Archival materials are unique.
The majority of the materials only exist within a given archive. Even when a collection has things like pamphlets or newspaper clippings that are duplicated elsewhere, these materials are unique in their relationship to other things in the collection. They are part of a one-of-a-kind set of materals that isn't duplicated anywhere else.
2. The vast majority of archival materials are not online.
Archives contain millions of pieces of papers and photographs, 1000s of linear feet (i.e. feet taken up on each shelf) of storage boxes, 1000s of hours of audio-visual materials, as well any artifacts the archives may hold. All of these materials have to be handled, sorted, and described before they can be used. In order to preserve the material and make it usable, archives place the material into protective folders and boxes. In addition, the things in each folder are all related to one another. Even if we had the time and digital storage space to scan every item, it would still be very difficult to make sure that each scan was findable and could be understood within its original context.
Instead, archivists focus their efforts on scanning some collections that they think lots of people will use, rather than trying to scan everything.