Very simply, the source ROM is code that isn’t readable in that way. The emulator interpolates the ROM binary. While ROM hacking is a thing, it’s done through hijacking the emulator’s/platform’s API calls.
Imagine having an audiobook as an MP3, but you want to rewrite chapter 8. You have all the words, you can hear them, but you don’t have the script. You have to transcribe everything first in order to have the book text. From there, you can edit what the book says, but it’d be easier if you just had the text in the first place.