Honestly? You really can't.
Maybe if everyone took the setting seriously, and played it very straight and reasonable, it might be a bit more believable. And there are small things that you can do.
Having your character view the planes and planar travel with awe. Viewing people who casually travel to the hells/abyss with suspicion and unease. Be amazed at the great feats that other characters speak of. Or maybe even doubt them(even if they're right).
But ultimately, this is a high fantasy server that has reached the logical extreme of what that implies. There are even more problems that aren't highlighted that add to the immersion problem. Including the following:
Transportation: With how many epic level mages there are on Arelith, and the existence of extremely convenient portals everywhere, this topic is completely circumvented. Who needs supply lines or any sort of road patrols? Just teleport everywhere, who cares. This is made even worse by the existence of caravanner NPCs who enable fast travel, but are never used because portals/lenses/boats/yoinking are just much more convenient.
Communication: Between speedy messengers, wisp bottles, illusions, and the parrot trinket, communication on Arelith is essentially a non issue. You can often see ridiculous situations occur when one evil person/monster is spotted upon the surface, and within five minutes, an army of thirty people has assembled because communication is instantaneous. There is a lot of potential RP that is wasted because nearly every epic level PC has a smartphone that has the entire island on speed dial.
Races: The bloat of adding more and more reward races, as well as the awful implementation of the reward system in general has led to this. Planetouched(Aasimar, Tiefling, Genasi), Half-Giants, Kenku, Gloamings, Avariel, Fey and Shadovar are all surface acceptable. I see numerous of these constantly. Even if you try to play a character that is surprised by rare and exotic races, you get worn down really quickly with how often they appear. This is offset even worse by the fact that there is no incentive to ever roll a reward character. So their population is only ever going to go up, unless they're mechanically suboptimal and have really uninteresting visuals/lore.
Economy and Inflation: This is a weird one, and something I haven't really dived into that much. But the isle of Arelith is essentially home to the most wealthy mogul millionaires in the overarching setting. There have been in-game books written on this subject, but just from a cursory glance, this is almost impossible to justify. Unless inflation has happened world wide, and one million gold is considered a normal amount for adventurers, there's really no way to justify poverty from an NPC perspective. Most good aligned characters would be able to uplift countless paupers and beggars into respectable living conditions.
Ultimately, you just have to take the server for what it is, and surround yourself with quality roleplayers.
Find a group that is interested in playing out a more grounded and realistic approach to these problems, instead of the cynical, setting dismissive response that is un-immersive.
There are some problems that will never go away however, unless the server direction is shifted that way and changes are made to combat those issues.
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Addendum:
To make this problem even weirder. I've often heard that an Arelith epic level character(level 30) actually translates to a level 14-15 character when compared to the rest of the setting.
With what our characters have access to, and the threats they face? This makes no sense.
If this is true, this compounds the immersion problem even further, by making our "level 15 characters" the most powerful and richest mid teen PCs that have ever existed.