No one was modding Oblivion before it came out. And PC version of Oblivion is required to create mods. Essentially.
TES CS package itself features only a few simple objects for demonstration purposes. You can't make a mod with those objects, no way. To mod the actual game, you need to load its master file (Oblivion.esm) into the construction set. More detailed explanations are found here:
http://cyrodiil.net/html/mods.htmlSomething more: making mods requires testing, lots of testing... how are you going to do this without Oblivion installed?