Either because it is a laptop CPU or (less likely) because it is an older Zen 2 might prevent it to reach the DRAM throughput.

Running the test on a desktop Zen 3 (5900X) with slower ECC DDR4-2666, i.e. with a maximum throughput of 42.7 GB/s, provides for 2 or more threads a throughput around 39 GB/s, with a maximum of 39.5 GB/s at 4 threads.