well just like any software not every task will consume all cores/thread, you can check this benchmark website testing different task of ue4 such as packaging, opening project etc : Unreal Engine 4.16 CPU Comparison: Skylake-X, Kaby Lake-X, Broadwell-E, Skylake, Ryzen 7
It is a little bit old doc but still show the different part of the engine using all cores or single threaded.
If you often do cooking, you might consider going for a lower cores count but higher freq. Unless epic make some modification or improvement on cooking workflow in the future(in ue5 maybe, who knows :d ) .