Because the 16 bit executables are closed source. :/ The special linker MILD.exe that organizes all of the RSP(Reality Signal Processor) microcodes and game data is the piece that needs to be reverse-engineered which is possible but currently beyond me. :)

I don't think it's beyond you, I just think you lack confidence. You can do it! I've been in your shoes before and it worked out.

Aww thanks much! :D Yeah I just lack confidence I guess since reversing binaries has always seemed a bit intense. I need to finally buy a really good code reversal tool and then start my quest to reproduce! :) It would be great if me or someone else was successful since then the N64 could be more easily developed for on any OS!!

You probably don't even need to reverse anything. Just learn how other (open source) toolchains work, read N64 manuals and information from other reverse engineers, and work incrementally. It's probably not necessary to reverse this proprietary ROM you're stuck with.