Because officially ddr4 is only up to 3200. See https://en.m.wikipedia.org/wiki/...

extropy on June 12, 2019 | parent | context | favorite | on: AMD Ryzen 9 3950X is the fastest processor on Geek...

Because officially ddr4 is only up to 3200. See https://en.m.wikipedia.org/wiki/DDR4_SDRAM

Every ddr4 module beyond that is officially a 3200 module with overclock option. That's why you need to enable Extreme Memory Profile in your bios to use speeds beyond 3200.