They want it as a standard option (pre-shipped with all decompressors). Not something they could incorporate into a customized fork / client that uses zstd with their own extensions.
Personally, as someone who doesn't work in web, I'm just as happy that zstd is flexible this way. For my applications, the brotli dictionary is pure overhead that bloats the library.
They want _every zstd decompressor_ to __already have__ the dictionary in question so that it can be specified as part of the standard. E.G. 'instead of empty / an initial in file dictionary, use the standard dict #3' Such reference dictionary starts would be not be included in .zstd files, but would be shipped with the compressor source code.