My best guesses. It has to do with the long incubation period. You can be infected and pass a test early on before you ever know you actually have the virus. This would allow you to travel and be social AKA spread the virus before you even know.
As for the atmosphere idea, I don't think so because my understanding is UV light kills the virus so I'm not entirely sure how it could survive in the atmosphere.
maybe this is a crazy theory but we know some people have shown cross immunity with antibodies due to exposure to some type of similar virus in the past. There are some valid empirical evidence this could be as high as 20-25% of the population. We also know the virus will flare up in a location extremely quick for 4-8 weeks and then go right back down just as fast. Wonder if the variants basically are just enough different that essentially those people that have cross immunity due one strain do not have that same cross immunity to a different variant. So essentially the strains/variants go through the population infecting those that have no cross immunity and then the next strain comes along and just is different enough that some portion of of that 20-25% no longer has that cross immunity and gets infected.