The main use of ROM is to store firmware, now you may be wondering “and what is firmware?” Let me answer you - firmware is kind of software that closely related to a specific hardware and needs updates frequently, that was the firmware short explanation now back to ROM again, as I said that the main use of it is to store firmware or application software in something called plug-in cartridges.Read only memory refers to hard wired memory. Most of the times it is impossible to modify the data of ROM without having advanced knowledges in the IT field. Not only for Nintendo, but for the publisher of the game being localized (if it isn't Nintendo). I can certainly see your reasoning on it having to be profitable. This may be why they've actually made changes to those more than anything else. N64 games being the most modern are also probably the easiest to alter because they probably have a lot more documentation and are easier to deal with in general because the game source likely still exists and can be recompiled. If the source doesn't exist you are limited to making only small changes to avoid breaking it which can be art or string swaps that are the same length as those found in the game to avoid shifting addresses. From there you run the risk of breaking it, or introducing emulation issues that need to be ironed out.

It has been speculated that the source code for these games may not still exist or if it does you basically need someone to go in there and figure it out which takes a lot of time and effort especially since older code will not be nearly as nice as it is today in terms of readability.

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