You are right to be confused. The idea that the simulators would necessarily have human-like motives can only be justified on anthropocentric grounds - whatever is out there, it must be like us.

Anything capable of running us as a simulation might exist in any arbitrarily strange physical environment that allowed enough processing power for the job. There is no basis for the assumption that simulators would have humanly comprehensible motives or a similar physical environment.

The simulation problem requires that we think about our entire perceived universe as a single point in possible-universe-space, and it is not possible to extrapolate from this one point.