Ryzen CPUs divide the cores into two "core complexes" that communicate over a bus called infinity fabric. Probably to make the engineering easier, the memory controller and RAM speed is the same as the infinity fabric's speed. You'll get good gains up to 3000 MHz, less so with more.
And the only thing I'll add is that "infinity fabric" is a singular clock-domain across all dies. So in Ryzen, its not a big deal cause there's only one die.
But in Threadripper (2-dies) or EPYC (4-dies), the "infinity fabric" bus is what connects the CPUs and Memory-controllers together.
But in Threadripper (2-dies) or EPYC (4-dies), the "infinity fabric" bus is what connects the CPUs and Memory-controllers together.