EY has two answers, one a bit less reasonable and one a bit more. The less reasonable answer is that he's a unique snowflake and diet+exercise does not work for him. The more reasonable answer is that the process of losing weight downgrades his mental capabilities and he prefers a high level of mental functioning to losing weight.

From my (subjective, outside) point of view, the real reason is that he is unwilling to pay the various costs of losing weight.

Your "real reason" is the same as Eliezer's second reason, except less specific.