There are just way too many reasons why, some legit some not so. It is not really fair to tar everyone with the same brush. I need to lose a few kgs as I am currently around 104kgs and 175cms tall. I dont eat a lot of so called junk food and just dont seem to shift much either way.
How can you penalise someone who puts on weight but does not eat much junk food and is fairly active?
There are better ways that can be achieved especially with junk food but our govts dont have the balls to implement. The single way they can do is this is to ensure manufacturers reduce sugar and salt content as well as additives and hormones or hormone like substances. This can be done without changing taste, it is just companies do not want to invest in doing this.
Our kids are not just getting fatter but also way bigger in general and a lot of this is to do with food additives. My 14yo is already about 174cm his 13yo mate across the road is around 185cms and another friend who is just 18 around 193cms.
So a lot of different variables.

