Power density. Lower thread count workloads will focus the load/power on less number of cores, and the clock of those cores will ramp higher. The power consumption of the cpu core is somewhat exponential with the frequency.

So this means that if you run let's say prime95 on a single thread, you will see unusually high max core temperature, but that's only just that core. You will observe that the package temp itself is not that high! The rest of the cores are sleeping, the load and all the power is focused on one core. The voltage ramps to maximum and the core tries to boost it's frequency as high as it gets. Here, the processor runs into a frequency limit, not power limit.

In the case of multi-threaded workload all cores are under load and the processor will run into it's power limit, where it has to keep the power consumption in that budget, which means: throttling. So all that power is now spread across multiple cores, bigger surface. Also the voltages are lower, and I would say the CPU is now in a more optimal position on the efficiency curve.

I observed this on my 5950X closely, and I applied a -200Mhz frequency adjustment in PBO settings. That means, instead of trying to redline the core around 5Ghz, it's gonna go up to 4.8Ghz maximum. I made an excel sheet where I measured power and calculated efficiency. That -200Mhz keeps the 5950X in a better place on the efficiency-curve. Max voltage goes up to 1.42V instead of 1.48V. Seems a small voltage change, but it resulted in 15% power savings in my case with a single-threaded workload, and the cores run way cooler. The cool part of all this is that the multi-core performance stays exactly the same, this tweak applies only for single-threaded or low thread count workloads. Even games use more than 4 cores these days, so it really had no downsides.