All ROMs do. ROMs are not human-readable; not by default, anyway. It takes time, effort and a fair amount of expertise to tease out the bits we want to edit.
Also, if you're working on a DS title, I suggest you check out The Console Tool
. DS titles are fairly standardized: they use an actual filesystem and can be browsed like a folder structure with the right tool. Low Lines' Console Tool supports most of the common DS formats; you should at least be able to view graphics and hunt down script files.