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.

It seems to be a really hard problem. Deep sleep even with Raspberry Pi Zero and Pico product lines has been problematic. I hoped they would make Pico 2 deep sleep better, but it still needs external deep sleep RTC like DS3231 to make it truly useful.