It's a bit unfair to say obese people are "just not trying". They might be trying very hard, they're just not trying hard enough.

Getting from being obese to an average weight is much harder than maintaining an average weight. Once you get yourself into an obese condition you've got a difficult road back out.

Though I agree with your basic argument - if you eat less than you burn you will lose weight. The tricky part is that your body can vary what it burns to some degree (your metabolism can speed up and slow down), so it's not entirely obvious how much you are burning.