It's not hard at all to get good uA/MHz numbers, it just increases die size and cost. You can push it pretty far by tuning the synthesis, for example the same Cortex-M4 core on the Ambiq Apollo3 Blue advertises "6 μA/MHz executing from flash or RAM at 3.3 V"
These are magnitudes of difference across the Espressif, Nordic, and Ambiq parts. The difference in price is also proportional.