Perhaps the first superhuman AGI isn't tremendously superhuman, but smart enough to realize that humanity's goose would be cooked if it got any smarter or it started the exponential explosion of self-improving superhuman intelligence. So it proceeds to take over the world and rules it as an oppressive dictator to prevent this from happening.

To preserve humanity, it proceeds to build colonizing starships operated by copies of itself which terraform and seed other planets with human life, which is ruled in such a fashion that society is kept frozen in something resembling "the dark ages," where science and technological industry exists but is disguised as fantasy magic.