[mlsu]

Most mechanical stresses in the drivetrain are transient and happen in the transmission. In the engine, they happen in the "out of band" power regimes -- very low and very high RPM.

The hybrid moves those transient "stresses," and the imparted maintenance thereof, into the electrical system. The engine never sees them.

This unlocks additional efficiencies. For example, the Prius uses the more fuel efficient Atkinson cycle engine. This is not possible in a standard car, because it cannot provide enough power in those transient regimes. You also get things like regenerative braking which help prolong brake life.

And, there's probably a bit of selection bias as well. I could see that the type of someone who buys a Prius is the type to not thrash it. That probably makes it appear more reliable in the statistics.