1. The cartridge could be a bootleg using a clean ROM dump of FireRed, but without the proper hardware for saving to function properly. Most bootlegs that actually work use SRAM for the save data (the type that needs a battery, which older Gen I and II cartridges used)... but in order for the game to know to use SRAM instead of Flash, the ROM needs to be patched. If it's a really cheap copy, then it's possible the pirates who made it just used a clean ROM.
2. The cartridge is legit, but the Flash memory area used to save (or the connection to it) has been damaged, rendering it invisible to the rest of the cartridge.
