Because MC was a human from earth that after he died, he was reincarnated in this other dimension. He manifested as a mage and when his powers grew enough, he gained the ability to read minds (This is where the story starts). Because reading minds is such a dangerous ability (it drives a person mad), our MC subconsciously manifested jelly and the "video game" interface in order to protect his mind. He said it's "like his favorite video game he remembers playing when he was a human on earth." They explain this in one of the earlier chapters and it sucks that you missed it because its REALLY important for the plot.
He's not "transmigrated" into a video game, all of his surroundings are real and NOT a script. He's really a mage in a different dimension from the past life that he remembers. All of the prompts from the interface were based on things his mind reading abilities picked up, not some pre-planned path. The "quests" were different things to help him hone and handle his new skill. Jelly (which is just his mind reading abilities put into an creature MC made) would pick up things from peoples minds, and then use the "interface" to communicate the necessary information to MC. If his magic abilities couldn't detect something, it wouldn't be a part of the "video game". This is also why Yanu was able to see the interface multiple times. And how the "switching" ability works at the end, he's not giving "control" of the game over, he's quite literally giving Yanu his mind reading ability, and it only works when both parties are deeply connected and trust each other.
So in short, if MC hadn't been from earth, his abilities would have manifested into something different than a "video game" interface, which makes the story way less cool.
No I get that. I’m just saying why include it at all that he got reincarnated from human to what became later a mage.
Like him reincarnating doesn’t really change his outlook on his life because he still grew up in the family he did, and lived in the “fantastical family he did”.
Like other than a couple lines where he said “wow that’s different from my previous reincarnation”, him being a reincarnation doesn’t serve anything and they could’ve just started the story as him being a mage from a well of family realizing he’s getting these video game like prompts cause he’s an ascended(ing) mage.
The whole reference to the "video game" wouldn't exist or work without him being from earth. His ability would be completely different and it would become a completely different story. Every scene with jelly and the interface would no longer fit. I'd argue its one of the MOST important facts for the entire plot. The video game references despite the world being "real" are what makes this comic different from every other fantasy yaoi comic out there.
I'm going to build on this comment and say that Eyra's personality definitely wouldn't be the same if he didn't remember his past life. While he is a bit strange right now and definitely adapted to the life in The Labyrinth, he still abides by basic human rights and cares about the people who live on his land, even when other people in power really couldn't give a shit, as shown in the story itself.
Besides, the comic doesn't really explain it well enough, but the novel builds upon the transmigrator. Yanu uses the reason for it to explain that dragons are quite literally inhuman and insane - because the interdimensional hole that allowed transmigrators and possessors to exist was created by a mad dragon that dabbled in magic (even while using magic is detrimental to dragon't regenerative abilities). And that was because he was convinced that his mate was in some other dimension - and he literally tore time and space to find that person and drag them to him (he succeeded btw, he and his mate were the ones who created the city The Labyrinth).
Besides, if Eyra was just 27 or however old his physical body is, he couldn't have mentally gotten over some of the things that happened as easily in the story. The fact that he is mentally almost 50 - his experience helped with his composure in some the situations and made it possible for him to be successful in the end.
Just my 2 cents.

Why even mention earth??? That wasn’t relevant in any shape way or form and I’m still confused ??? Help ????
Still pretty good read, should read if you’re bored but don’t expect revolutionary story writing. But diffuse to follow until like chapter 35, then towards the end it was peak.