Part of that is down to manufacturers, who especially in the US put loads of sugar in pretty much everything. Part of that is down to the consumer, which chooses convenience over awareness; part of that is things like take-out or drinking culture, but part of that is also diet culture, a diet telling you "stop eating this, only eat that". That too is convenience.
As is calling certain foods "healthy" vs "unhealthy", because that's no absolute either, and it's a lazy descriptor - as in, "please explain to me why it is healthy / unhealthy". The crux of it is, nothing is healthy or unhealthy unless it's literal poison, it's more about calorific content and quantities.