It’s because ram is even faster with lower latency.
Pcie4.0x4 nvme is 40Gbps (I presume you mean pcie4.0 which is the newest and greatest over pcie3.0).
And that’s if it can actually sustain that level of read/write consistently, and isn’t just dumping data into a buffer.
DDR3 1333mhz is 80Gbps (which is 15 years old).
DDR4 2133 is 136Gbps.
These are just rough numbers. Actual throughput is going to depend on number of channels, mobo, CPU etc.