Game cartriges are grown on a farm. When they are made, all game cartriges are flashcards, and can be any game you want them to be. But as they grow old, they start to get rigid, just like your parents. Also, arthritis sets in. In the end, they become so petrified, conservative, stuck in their ways, they are stuck with one game, and one game alone, and they refuse to change, no matter what. those cartriges are then harvested, packed with an instruction booklet, and sold in stores.
ROMs are the "psychological profiles", done by special profilers, that determine general personality types. All game cartriges fall into one of these types, and the individual differences are called "savegames". Those personality types are charted, defined, and then posted online for further study by a wider community. Thus far, there are 2723 defined personality types of GBA cartriges, and 950 types for DS cartriges. Scientists are finding new ones every day.
Some theories claim pop culture and mass media affect the cartriges and affect the creation of new personality types; popular movies, trends and even toys seem to generate whole new personalities. Such examples are profile GBA2700, "Ghost rider", evidently affected very strongly by the movie "Ghost rider" (can't say why), or type NDS575, "Bratz Forever Diamonds", molded by the "Bratz" series of dolls and accessories.
Flashcards are actors. You give them a ROM profile, and they act it out.