Alternate explanation: ratfic tends to come out this way because it prioritizes a certain kind of economic / game-theoretic realism, and in the long term multi-polar equilibria just aren't that realistic in many settings (including, IMO, our present non-fictional setting...).

I generally think rationalists gesture at this as inevitable more than they actually demonstrate that it is inevitable; i.e., long term multi-polar equilibria has been quite sticky for Westphalian states or for plankton.