It's not clear, although it seems reasonable to assume that the availability of cheap processed food is a major factor.
If changing one's environment were a straightforward thing, like changing what foods one keeps in one's cabinets, then the data on the effectiveness of weight loss interventions would not be so dismal. An effective change of environment is probably more like "moving from Alabama to Japan".
What specific aspects of our environment are you considering? To some extent one can change one's environment.