If you've completed enough endings of Nier Replicant/Gestalt you'll already know the twist at the end of Nier Automata, but less known is that the triggering event of Nier's plot, White Chlorination Syndrome, is introduced by ending E of Drakengard, where the Japanese Air Force shoot down a dragon that just killed a god in a dance off.
This means that the reason you can find the Drakengard weapons in the Nier games is because Caim was carrying all 65 different swords, axes, maces, hammers, spears, staves, and polearms on his person when he was blown to fucking bits by an air to air missile, scattering them across Tokyo.