Because it's more convenient to use workshop tags. There are addons with similar functionality that scan each mounted addon for a certain file extentions. Addon Share looks at the model count of the addon - property from Workshop addon metadata.
Workshop guidelines state, that a single addon must contain every textures and sounds it needs to function without errors. Lua is handled by the game itself, and the current map is added to load order by the game as well.

For your server, either pre-generate addWorkshop() lua file for your collection with online generator ( https://beta.configcreator.com/create/gmod/resources.lua ) or use WSDL addon, which scans your mounted addons every restart