What makes you say this? There are plenty of examples to the contrary. Airlines, cars, gas stations, shoes, beer, bikes, browsers, power tools, pizzas, etc.

Sure, there are plenty of duopolies out there, but there are also plenty of monopolies. That doesn't mean they're inevitable.

This may be the illusion of choice provided by disruption... Eventually all would settle to just two if no further disruption (periods of chaos) challenged the natural order of dominance. Not feasible in reality as change is constant. Just in theory. Occasionally we see a clear bifurcation the more order we nuance out in large complex systems like politics where the stakes are high and attention is great. We settle on left or right. The sage walks two roads in this case.