The Raspberry Pi SoCs optimised for cost rather than power consumption (or maximum performance). This is an engineering trade off, not a criticism. More power partitioning requires more area, more complexity in design and verification, and more expensive external components to support the internal voltage domains.

Another comment mentions the RP2xxx microcontrollers. If you look at those, they are optimised for compute power and data throughput rather than low power operation. I think it's a reasonable choice - the Pico boards are pretty sturdy and the original target is people running MicroPython, Arduino, etc rather than looking for µA standby currents.