Ever since Legion and the acquisition of a space ship that can orbit the planet, it’s getting very difficult to believe that there are any landmasses on the planet that we shouldn’t know about.
So with every new expansion (like War Within), I’m finding it more and more difficult to believe that we all just missed this giant landmass. In reality, the Alliance and Horde should’ve already sent exploratory missions to the other side of the planet for resources and information gathering.
Well, with TWW, we are mostly going underground, which is pretty believable that we didn’t know about, even with a spaceship.
Hidden by magic. This was already explained.
Basically not that different from Pamdaria, shrouded in mist.
The land for War Within is a small island and explorers disappeared going that way into the forbidden sea.
Blizzard is playing gods!
When asked about the software we were working on an old friend of mine use to say “The truth is in the code”. When it comes to computer programs, the developers are the gods.
Azeroth exists as a data structure in the WoW software. To add more continents you basically turn "0"s into "1"s.
So far most of them have been areas that were previously established but not available ingame. Pandaria and the Dragon Isles had lore reasons they couldn’t be visited previously, but Kul’Tiras/Zandalar and the Broken Isles were just places that existed and characters had visited in lore but we just couldn’t go there ingame.
I believe Warthin is the first completely new area and they’re justifying it by having it be underground.
Often it’s well secured such as the mists covering Pandaria or the Zandalari fleet. Mechagon was left a mystery against the Kul’tirans because vessels or gryphons would get attacked on approach. 
In most cases, the landmasses are known about, but we have no reason to go there, so we don’t.
In the cases where landmass is “missing” or “lost,” it can probably be assumed it’s perpetually covered in clouds, fog, or possibly surrounded by storms, which at least partially obfuscates it from explorers and spaceships.
As characters, we probably wouldn’t care about everything we do from space. So there is this island… we have no reason to go there so why care about it until we do?
However, so far, everything added is a place already rumored to be out there in a book or lore snippet somewhere. So people do know about these places before they are added to the maps.
Can’t wait til we can finally go to Tel’Abim. Their bananas been hyping them up since Vanilla.
I think TWW has some loose lore ties like Dragonflight’s Dragon Isles and BfA’s Zandalar/Tiragarde.
There’s one zone that I recently stumbled into a lore video of which doesn’t exist in-game but I expect will also be shrouded in mist or the like if we do get the zone. I don’t remember what it was called, just that it started with an A. From a lore standpoint, hiding the zones is a nice and easy explanation for why we never went there before. From a technical standpoint, if Blizz needed to launch WoW in Vanilla with every location in their lore, we would never have WoW. Not to mention, their lore changes and adapts over time (some of this is bad, like the Shadowlands retcon-fest, but some makes for interesting story developments).
I always was amused when a new continent was added/“discovered” right in the spot where the ships we used to travel between the early continents would pass directly through.
It’s also amusing how the early maps compare to current ones and you realize not only were there large landmasses we were missing, but sometimes the early continents moved to accommodate the new ones on the map. Plate tectonics on fast forward.
Also, maybe Azeroth is still growing, adding surface area for new places.
/moo 
By opening up the map and drawing new landmasses.
Another idea: do what Christopher Columbus did and sail around the other side of the world. We have not looked east of Eastern Kingdom or west of Kalimdor.
I don’t know why they haven’t added some of the old RTS map landmasses like Crestfall and Northeron. I for one wonder what’s happening in Tel’Abim other than banana harvesting. Maybe in Midnight?
Tectonic shifting? Magma hot spots?
Isn’t Azeroth (the Titan) at the world’s core? Maybe when it moves, rumbles, burps, mass is forced to the surface.
And the excuse with DF was still dumb.
Pandaland it was fine because it was the first time they did it, but literally the same gimmick as Pandaland done twice is insane.
To be honest I was fine with both Broken Isles and Zandalar / Kul’tiras because those were known locations, but DF is just super dumb from a lore point of view. I don’t care if Dragon Isles was something they wanted to do since Vanilla, if it was going to exist it should have existed already, now it’s just goofy.
Dragon Isles was hidden by some sort of magic.
Not hard to miss then.
If there is the rumored other side of the world its also likely being hidden in some way.
Gnomes with really big BULLDOZERS!!!
Like a D9045.
Enough room for new landmasses.
The map the way it is could be zoomed out at some point to reveal more water where new landmasses could appear in the future and there could also be more planets like Outland.