Why are people running surpluses?

Food is much more readily available and convenient. Look at all the food you can buy. Then look at how much of it can be stored at room temperature, needs no preparation, and has a long shelf time. Then add in all that you can buy that stores in your refrigerator, does not need to be prepared, and has a long shelf life. Lets take it further, look at all the food you can buy, that you store in your freezer, that you prepare by sticking it in a microwave for five minutes.

The abundance of food combined with an excess in idle time, namely the result of a forty hour work week or no work week, leads to over consumption. I always love the farm comparison. Growing up it was eat before light, a big lunch, whatever you grabbed off the tree or stole from the garden, and a big dinner. As a kid if we weren't doing chores we were running off with friends to a game, fishing, or forever building that fort. These days its inside till mom/dad gets home, food everywhere, and an XBOX/PS4/Etc.

Then go look at the people you work with. Its pretty easy to tell why some are overweight. Lunch out every day at the fat festival around the corner, that place that serves 1200 calorie or more lunches for seven bucks. Usually a good chaser for that double donut and coffee with cream breakfast (or bagel with cream cheese). Then those little snacks in the desk.

Its simply calorie overload with nothing but idle time. Hell these same people will make fun of the fitness/diet "freaks".

TL;DR

Idle Time, Endless ready food, results in calorie overload

In a practical sense, the question doesn't matter at all to people trying to live healthier lives.

The reason we have fatter people is we have more calorically dense foods readily available (meaning greater calories in) while people live more sedentary lifestyles (means less calories out).

This function to lose weight is so incredibly simple (paging jacques chester and his steam machine blog post).