Maybe it's more like a bubble around Argus, so there's nothing else around that can be seen from Azeroth.
Less complicated answer: Because it's a texture overlaid on the skybox and they didn't bother with an animated portal effect or a background with whatever's behind Argus.
/wave to Sargeras since Argus is his vessel like Azeroth is a titan planet, would be hilarious to see 2 worlds going at each other.
I see 2 possibilities, either we fight Sargeras and Azeroth helps us or she does get corrupted like Sargeras fears and he helps us defeat her then we fight the void lords together or we uncorrupt her first then both of them help us defeat the void lords.
Or... they took the transformers idea and Argus did move in orbit past the portal and then closed.
Sofar we only have seen Legion use keystones to move ships and then the portal close behind them.
I dont get why this keystone keeps a portal open forever on a planet size scale.
From a story-writing point of view, it's way easier to open and close a portal without consequence than it is to place and remove a planet next to another without consequence.
I'm 99% sure we won't have that thing floating there after Legion, and I very much doubt Blizzard is going to make an event about pieces of Argus falling on us after we blow it up or whatever. Unless they're going for that "Argus is rising the tides and the naga use it to attack us" theory, I say it's just a portal.
Not the first one, either.
Shath'mag vwyq shu et'agthu, Shath'mag sshk ye! Krz'ek fhn'z agash zz maqdahl or'kaaxth'ma amqa!
The Black Empire once ruled this pitiful world, and it will do so again! Your pitiful kind will know only despair and sorrow for a hundred thousand millennia to come!
Avatar drawn by Sir Meo
Shath'mag vwyq shu et'agthu, Shath'mag sshk ye! Krz'ek fhn'z agash zz maqdahl or'kaaxth'ma amqa!
The Black Empire once ruled this pitiful world, and it will do so again! Your pitiful kind will know only despair and sorrow for a hundred thousand millennia to come!
Avatar drawn by Sir Meo
It feels pretty likely, given what we've heard about Azeroth and the implications about Argus (like the 7.2 cinematic), but I kinda hope it isn't. On one hand, this expansion IS called Legion, and having the main dude of the Legion stay alive seems strange, but on the other hand we'll have killed all the strongest lore characters from before WoW.
Shath'mag vwyq shu et'agthu, Shath'mag sshk ye! Krz'ek fhn'z agash zz maqdahl or'kaaxth'ma amqa!
The Black Empire once ruled this pitiful world, and it will do so again! Your pitiful kind will know only despair and sorrow for a hundred thousand millennia to come!
Avatar drawn by Sir Meo