Smart Folders are not folders in the file system, they are a display of search query results. Smart Folders (or Smart Albums in Photos, Smart Playlists in iTunes/Music, or Smart Mailboxes in Mail... ) are all search engine results. When you Save a Smart Folder, the default location will be the Saved Searches folder in your User account Library folder.
Search queries are very economical to store - just a few lines of text. The results of a search query can be huge, so duplicating those results as physical files (rather than viewing a list of search results) is very uneconomical.
While Smart Folders could be an effective way to make a targeted sync/backup of a particular data type, they also carry certain risks, as there's no limit on the number of queries that might include the same physical file. The key risk is redundancy - saving/updating that same file many times. If that one file was in only three Smart Folders, that's three times the bandwidth, three times the cloud storage, each time a change was made to the file.
While it certainly seems conceivable that a product like Google Drive or Drop Box could be written to sync with a Smart Folder, it would have to be a conscious decision to write the code to do that, and good reasons for them to ignore Smart Folders' existence altogether.
