How did the people in the Naruto Universe forget about their history?
Naruto/Naruto Shippuden is mainly focused on the tale of the gutsy, unpredictable, strong willed protagonist ninja Naruto. Everything that relates to him is shown in the story.

Of course, the sage of Six Paths taught people the Jutsu. An art of fighting to protect themselves or the weak to retain humanity on earth. The human race were then able to use Justsu to at least fight against beasts/animals (for example, the ten-tails which attacked)

People later started to misuse Jutsu, stemming from their hunger for power and wealth.

The history prior to Rikudō Sennin is not important to Naruto's story so it is not mentioned anywhere. That doesn't mean that the people have forgotten everything.