Also curious what MCUs you're working with to give you this impression?
RP2040 is 264k, RP2350 is 520k.
I use NXP's rt1060 and rt1170 for work, and they have 1M and 2M respectively, still quite far away from 16M and those are quite beefy running at 500MHz - 1GHz.
While I generally agree with you, the RT106x line does support external SDRAM as well. I've got an MIMXRT1060-EVKB sitting here on my desk that has 32MB of SDRAM alongside the on-die 1MB of SRAM.
Those specs are $50 for compute module - a very non-trivial cost.
> NXP's rt1060 and rt1170 for work
These both have FlexSPI controllers capable of interfacing with $3-5 in PSRAM at 8M or 16M.