I does not only happens with modds from Internet but also with scennarios already charged in the Steam Civ3 download.
A lot of the (CFC-)user-made mods supplied with Civ3Complete were basically just the .bic/.bix/.biq files (sometimes with a readme.txt), which required multiple additional file-downloads to become playable.
Unfortunately, in many cases, these downloads were made available from personal file-hosting websites which have since been shut down. And of the Civ3-Mod zipfiles that
were uploaded directly to CFC in the early years, after release of the PtW (2002) and Conquests (2003) expansions, many were also lost in the so-called "Great Hack" of 2006/2007(?) (not sure of the exact date, but it was well before I joined CFC).
Your best bet to fix any "lost" mods that you're interested in, may now be to ask in the
Creation & Customization section of the Civ3-forums, whether anyone has a working link to the complete versions at e.g. Google-drive or similar, and/or would be willing to re-upload them. Once your postcount is a little higher, you will also be able to "Start a Conversation" with (=send private messages to) other CFC-users, although obviously there's no guarantee that any specific mod-creator is still active here.
But as well as
@Civinator, several other CFC-users may also be able to help you (if you ask them nicely!): off the top of my head,
@Node60,
@Ozymandias,
@Blake00 and
@Takhisis have all been able to help out with similar missing mod-file problems (mine and others) in recent years.