Your stomach stretches out after a long enough time of eating like this and you have to eat even more to feel full. Pair that with the availability of highly caloric foods designed to make you crave them and it's a recipe for disaster.

Watch some "my 600lb life" and it starts to make sense. It's a slow boil, to be sure, but I think it can happen to anyone that isn't paying attention, who uses food and that feeling of fullness to cope with whatever negative feelings they have. I like Sean of steel (former 600lber), he gives some really good context in his react vids on youtube.