The ARM SoC (system on chip) CPUs these boards use often cap out at supporting 4GB of RAM. It's also much more expensive and power hungry to stuff a bunch more DDR4 onto a tiny board, and often messes with the layout (i.e., of you need two RAM chips instead of one).

The rk3399 CPU on this board also dates from 2016[^1] so adjust your expectations accordingly. :)

[^1] It's also a well-documented chip that's cheap, provides a nice performance bump over an RPi, and requires fewer proprietary blobs.