I suppose it's because it's not clear to me how herd immunity will present itself in terms of testing data.
I'm expecting to test positive for some time having had covid a few weeks back. As you say, plenty of asymptomatic folk may also test positive but have some immunity to serious illness due to previous infection / vaccine. Perhaps these people are also less infectious, preventing the onward transmission?
I dunno. All I do know is that if hospitalisations fall during a period with no restrictions in place, something pretty seismic has changed. This has not happened before. All the other data might help understanding of how this could happen, but hospitalisation remains the key metric in my view.