Cuttlefish, while it may be a more effective reference device, just doesn't accomplish the same thing because Pixels were used for more than just as a reference target (e.g. GrapheneOS).
Plus, there's just something cooler about running your own build of Android on real hardware v.s. a VM.
The article mentions that they're also supporting GSI's as a reference target of sorts, and that's way closer to real hardware. GSI's are annoying for other reasons though - for example, there isn't a single "GSI" build type, they vary according to low-level device features (such as partitioning) and what version of Android they first came out with. Still, it's better than nothing.
These days there's also GKI, a "generic kernel build" (minus custom modules and blobs) that's supposed to work on any recent device. Note, this is not a "mainline" Linux kernel at all, it's still very much a downstream fork with lots of custom patches. But it too is supposed to enable testing and development in a unified way, regardless of the actual device.