I wonder if they're particularly good at balancing the cells. When people recycle 18650s from laptops, I think it's often because one or two cells went bad while the rest remain fairly healthy.

Similarly, if you put things like batteries or capacitors in series, you typically want circuitry to balance their voltages, because a pair of 2.7V elements charged to 5.3V could really be at say, 2.75V and 2.55V, which would be unhealthy for the former cell. I think those imbalances are also caused by small differences in the individual elements' properties (capacitors are typically less precisely-tuned than resistors) which will drift further over time with wear and temperature changes and overvoltage conditions and whatnot. So that 2.75V cell would degrade very rapidly compared to the 2.55V one, probably causing even larger imbalances until it eventually fails.

Most lithium cells have basic protection circuits built-in against over/under voltage and overcurrent (e.g. shorts,) because they tend to explode if you leave those things out, but they definitely last longer if you pay attention to them as individual cells rather than a system.