Board Thread:Sonic News Channel/@comment-11463617-20150307025432/@comment-679780-20150307130550

Officially he's always been named Dr. Eggman... in Japan. When Sonic was introduced to a westenr audience in the 90s he was renamed Dr. Robotnik in the manuals.

Eventually Sega passed ownership of Sonic from Japan to America and for the Dreamcast era bits of Sonic's lore has been rewritten to say that Dr. Robotnik is indeed his real name, as confirmed in Sonic Adventure and continues to be his family name in Sonic Adventure 2 as Gerald Robotnik is confirmed to be his grandfather. While in Sonic Adventure Robotnik silenced Sonic for calling him "Eggman" the names simply became his moniker, as well as calling his empire after the name.

Perhaps he is simply aware he simply resembles a large egg to argue against it.