I've posted about this many time as I'm on NHS Obesity prevention group (in a professional capacity)
It is an interesting question but there has been so much research around what causes obesity that this part is fairly well documented. What to do about it is another matter and what we discuss with various agencies.
The number one contributory factor is poverty. If you are poor, you are significantly more like to carry excess weight. Many reasons for this but we all know processed food is cheap, it's also quick & doesn't required much preparation important as you're more likely to live in a working and/or single parent family or have parents working shifts. If you live in a deprived area you are also more likely to be exposed to higher number of fast food outlets, have less access to outdoor space, unlikely to be as physically active, more likely to have mental health problems, more likely to have a long-term healthy condition....it goes on and is thoroughly depressing.
Other factors: genetics, where you live, injuries or illness, ethnicity & cultural norms all of course play a part, environmental factors, childhood experiences, (for example neglected children often have complicated relationships with food).
People don't just wake up one day and decide to be 'greedy' or 'lazy' (inverted commas as this is not a phrased I'd ever use and so rarely the case).