But it's largely because today's improvements have gone into lower CPU consumption than core count. But it could have gone core count, it just 99% of users wouldn't know what to do with more cores and would rather take the battery life.
Fixed that for you. Remember that every time you experience an unresponsive GUI anywhere, and that's the easy case.
Beyond that, parallelization is generally not easy.
E.g. Qualcomm SDM845 has 8 cores in total, 4 faster ones that are only used rarely, and 4 power efficient ones.
Also they offload increasing amount of work from less power efficient general-purpose CPUs to other on-chip hardware, DSPs (for many years), and now GPUs (marketing folks call them neural engines or AI chips).