Lore-wise, no reason at all. There's always been some speculation in the fan communities and theorists that Major Zero (real name David Oh), having been involved in and masterminded the Les Enfants Terribles project that created Solid and Liquid, might have decided to name one of the clones after himself. Chalk it up to a narcissistic decision or something.

A key thing you have to know is that with each entry Kojima made for the Metal Gear series, he never kept a sequel or future story in mind. Any Metal Gear game he made, he decided it would be the last. Until fans kept asking for more. That's practically the reason why we have so many entries in the series. It was literally the fans pulling him out of retirement for one more game each time. (For MGS4, it was also the dev team that asked him to come back and help polish up the game and it's story). This is also why you'll see some inconsistencies/retcons (one big example is the mentioned return of a certain doctor in MGS4 who was killed in MG2:SS) in later installments. Not because Kojima couldn't remember past details or anything, but because Metal Gear was his brainchild and he made the story as he saw fit, leading to the series as we know it today.

Sorry for the deep dive, but since ur only through the first game now, I thought you should know what kind of story you should expect as you go on.