The contents of WBFS files are basically an image of the actual game file content on a Wii disk. Any extra information from the disk has already been removed. Additionally, developers will generally compress their game's resources in order to reduce storage space and reduce required streaming bandwidth from disk.
Given the above, using .zip archives of WBFS files would not save you much space at all, so you are not missing out on much here.
Zip specifically isn't possible, because zip to my knowledge doesn't allow random access, files have to be decompressed starting at the beginning. But there are other formats that do. There's the ciso format supported by wit, which does support it AFAIK, but cIOS does not. Scrubbing is the only thing that saves space while still being able to load the game on a real Wii, and that's already done when converting a game to wbfs.