I think it's spreading because it is a highly contagious virus that we have no immunity to. Whether a big portion of this is because of a few prats in supermarkets not wearing a mask properly is probably down to your outlook, because I doubt there is any evidence for this being a route cause. You can be wearing your mask perfectly but touch your forehead while looking at a tin of beans then putting the tin back, well there is potentially the virus ready for the next shopper mask or not...
It's a lot more than a few prats. And what about the non mask wearing idiots who don't social distance and gather in large numbers to protest against lockdown measures? Not the only cause, I'm sure, and to suggest that I think it is the sole cause is pretty ridiculous - but non mask wearers with symptom-less Covid are thoughtless irresponsible super-spreaders.