How's that? If someone says "why am I so overweight?" do you say that it can't be the eating because that hasn't changed?
If you've been eating the same for the last 20 years and you've been gradually gaining weight over the last 20 years, then it's your diet.
If you've been eating the same for the last 20 years and you suddenly gain weight, it's likely not your diet, it's something else -- exercise, illness, et cetera.