The Momonga we see in the new world (new game) is an AI version of that player. Why bother with the far future? Why the differences he has noticed in his emotional make-up? My theory? He isn't "real". Free beta testers! – RichF
That is a great answer! It also explains so many other things, for example, the new setting appears to run on many of the same basic rules...with new options added, like marshal arts (and rune magic) that didn't exist (or weren't implemented, or were largely unknown to the players), in Yggdrasil. Also, there is mounting evidence...we even see plenty of it in the anime, that there have been other players in the new World...many, many years ago.
Momonga could have been sitting, as lifeless data on a hard drive, along with the rest of Nazerick, for all the years it took for true AI to come into being, finally, after loads of history takes place, often using rules that came into being long after Momonga was recorded, the Great Tomb is awakened and the Supreme One orders Sabas to scout the perimeter...
If he were still thinking using his human brain, back in his apartment, he would have passed out from low loos sugar and lack of sleep days before he even goes outside for his fateful Moonlight talk with Demugure.
On the other hand, maybe it is all just magic. Maybe when the Wild Magic event that created the currently popular forms of magic occurred, it copied the rules for how the system of spells would work from a computer game on a different World. When the game was copied, it was copied complete with the popular dungeons, like Nazarick. Another popular story, in the same genre ("Isekai"), which should also get an anime,"Kumo Desu Ga, Nani Ka?" (high school class becomes heroic princes and princesses...our hero, friendless gamer-girl becomes mook tier monster in a dungeon) eventually deals quite well with the often ignored question of why a real-seeming World would so closely resemble a game. I, personally, am very fond of how "Re:Monster" (govenment assassin becomes goblin, in goblin clan) handled it, too.
It is very, very common, right now, in Isekai stories, to have the new world the hero is reincarnated into be bizarrely similar to a "RPG" video game, to the point where everything runs on "Dungeons and Dragons" style rules, even when they seem ridiculous for a "realistic" fantasy world...when they seem like an approximate model of reality, that skips a certaint level of detail, rather than anything that would ever naturally develope. While this seems really odd to most readers/viewers who aren't used to it (English speakers), it is so much a convention, that the vast majority of the stories don't even question it, some of the best ones do. The two I just mentioned, and others; "Kono Subarashii Sekai ni Shukufuku" is a parody, and acts like a parody, "Youjo Senki" has divine intervention as part of the basic premise, "ErfWorld" had a magic spell looking for someone, somewhere in the multiverse who had a required aptitude with the way the World worked (so it grabbed a game designer who had made a similar game)...SAO tries to explain why people are "trapped in a game" and is EXTREMELY popular...but I feel it falls far short of Tad Williams' "OtherLand," but apparently popular audiences wanted a quick explanation, rather than clues dribbled out over five long novels.
Overlord might just leave the question unanswered, as so many do, and we will have to decide for ourselves. Momonga is certainly better off as Ainz-sama, than as "Sezuki Satori" in his crapy, distopian Japan, and I'd enjoy his story, in his New World, even if he, and we, never find out how it happened...I just hope we get more than just 12 new episodes, in this story (second season is happening, I'm so happy) because our World really needs the few truly great isekai stories, of which Overlord is definitely one, to become well known...because there are quite a few stories that aren't as good, and the genre is very popular, in some languages. The potential audience needs to see that modern popular isekai can be great.