That works in places that kept good records, but I have to imagine the vast majority of counties don't have those records. Plus I'm sure many people got infected and stayed home with mild/moderate cases and never got tested (because they knew they had it and/or didn't want to risk spreading to others), so there would be no record for them anyway. Or their cases were so early on in the pandemic that there were no tests available or the ones that were available were very unreliable (I remember the early ones only had ~50% accuracy)