Its not something you allow to happen, it's something you do on purpose. Most people have to force feed or at that level ignore their body's leptin signaling that tells them they're full.
Take the average Black man (statistically 70% of us are overweight or obese) have him eat an extra BK Combo and a slice of cheesecake every other day. that's an average additional total of 2000 calories. If he does that every other day it's an average weight gain of 2 lbs. And a total gain of about 100 lbs a year.
An athletic person could look at the average American's body and ask the same question OP asked "How did you allow this to happen?"