If individual virons really just float around and infect others that way, it would mean everyone with covid would have been spreading it everywhere they went like walking crop dusters so everyone in the US would have been infected in the first few weeks of the outbreak.

I would not completely dismiss that possibility. I have suspected for some time that people who were not symptomatic when they were first infected could become symptomatic when they are infected for a second or third time if the virus is really airborne HIV and there is a lot of circumstancial evidence to support that it is, not least the research that showed SARS-CoV-2 infects CD4+ T Lymphocytes and the glaringly obvious lack of mainstream discussion about that.