When the limiting factor for IO perf that you’re trying to measure with your benchmark is the filesystem, rather than the disk, you don’t actually want a real disk backing your filesystem; that would just introduce noise to your measurements. Instead, you want your filesystem to be backed with an entirely in-memory block-device loopback image or some equivalent — which VMs are perfectly capable of (and in fact better at) providing.

Think of it by analogy to e.g. GPU benchmarking: you’d never use anything slower than the fastest CPU you can get your hands on, because you want to benchmark the GPU on its own as a single system bottleneck; not how well the GPU idles when held back by a bottleneck somewhere else in the system.