system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why could retro cartridge-based games not save player data without a battery backup? Why do modern cartridge-based systems not let you do this as well?

I assume with retro systems there was some kinda tech limitation. But if the game data could be saved on the cartridge why could a small rewriteable section of the cart not allow player data to be saved on it that wouldn't require a battery? I know batteries are used to hold data in volatile memory on things like cartridges but yeah why could you not save to Non-volatile memory?

Furthermore with the advancements in storage and us now having SD cards the size of ones fingernail why cant player data be stored on a a Nintendo 3DS/Switch cart? (...I assume, I dont actually own either.)
gpt
Technology like the Non-volatile memory in SDcards didn't exist yet, or at least not in a commercially viable form. If the classic NES was made today the system would contain either a hard drive, an SDcard slot, or a portion of non-volatile memory on the motherboard.

The Cartridges used on older systems like an Atari or Nintendo were ROMs. Read-Only-Memory

The chips were created at the factory without the ability to be modified so there was no way to store dynamic information on them.

To get around this cartridges would have a small amount of NVRAM (Non-Volatile Random Access Memory) that could store variables (like what level you are on, your life total, which weapons you unlocked, etc) even if the power was shut down.

NVRAM can only keep its contents if it has a constantly power source, which in the case of Nintendo games was a small watch battery inside the cartridge.

> Furthermore with the advancements in storage and us now having SD cards the size of ones fingernail why cant player data be stored on a a Nintendo 3DS/Switch cart?

They total could, they just don't :D

There is no reason that Nintendo couldn't use common SDCard technology to release video games for a portable system, other than they don't want to. Probably for anti-piracy reasons.