How much of that is because hybrids put an easier load on their engines and how much because Toyota knows how to build a reliable vehicle? It's both, but I imagine it's mostly because of Toyota reliability.
In fairness a Prius has no clutch, no torque convertor, no starter motor, no sliding dog clutches, no syncos, no turbo etc. It's unfortunate that people understood the Prius system as a normal car plus the electrical motive system but the reality is actually not nearly as complex.