Car companies already travel all over the US to test varying parts of the vehicle, with individual teams ending up in places like death valley, alaska, or north carolina's tail of the dragon. Why would they not do the same for autonomous testing?

Sure, they would. But they've done most of the initial development on dry roads. Snow driving is rather a different beast, and has had less development and testing time (because the first step is to get it to work at all, in the best conditions).

Sure, they'll test it on snow. And I'll probably trust it on snow, eventually. But I'll trust it on snow later than I'll trust it on dry roads.