NES games didn't have a spritesheet, there's no image stored from where to read the sprites.

Quoted for Emphasis.

Basically everything before the PS1/Saturn/N64 had no concept of an image or texture -- just some bits that fit into a handful of fixed-size buckets. When you see sprite-rips of these old games, that's just the people making them transforming them into a familair format for modern uses. Color values and color depth you see in these files are similarly the modern abstraction of how the original hardware would have understood those bits together with the values written to the hardware color palette. Early PCs, Amigas, Atari STs and the like worked something like a reasonable facsimile of how modern games would operate, but earlier computers and game consoles up until about '95 worked very differently.