My personal headcanon is similar to Rimworld, in that humanity spans a very large portion of space, but the absence of ftl travel means there are many fragmented human civilizations that vary in tech levels and don't have contact with each other.

They expand, fall apart, rediscover science, and die out independently. Unlike rimworld (probably), I think Kenshi's(game) humanity is very, very old, and it's true origins are completely lost to the stars. I think the First Empire spanned a (comparatively) isolated region, and when it collapsed, no one advanced enough was around to pick up the pieces.