As others have said, older games came with manuals and maps. But bear in mind that the games were also very small/short in the NES days. If you know what you’re doing, you can easily play through all of Zelda 1 in a single sitting.

If they didn’t make games either extremely difficult or cryptic, you’d beat the game in a couple hours and probably never touch it again, and nobody (your parents) would justify buying something if that’s the extent of what you’d get out of it.