On a modern desktop/server CPU, the RAM memory and PCIe device mapped memory do not share a bus. The equivalence is a fiction maintained by the MMU. Some chips (e.g. Apple Silicon) have unified memory such that RAM is accessible from the CPU and devices (GPU) on a shared bus, but this is a little different.

Also, direct access of device memory is quite slow. High throughput usecases like storage or network have relied entirely on DMA to system RAM from the device for decades.