1. ...And this means that the other continents with their different histories and culture don't have things to offer?
Not that they don't have things to offer. But you're missing the point. Tamriel is the limelight. In terms of the overarching fragmented plot that is TES, Tamriel is where the center stage is (from both in-game and out-of-game perspectives). It's where everyone wants to be. It is the start of everything, and hypothetically the end of everything. It's called Dawn's Beauty for a reason. Adamantine landed there for a reason. It receives so much mythical attention and trouble for a reason. Therefore, in the sense of the direction TES as a story, it's not going to go off and explore the tangential cardboard backdrop that is Akavir.
2. Because when two parties clash multiple times the outcome is always the same, right? Look at the ultimate fall of Rome. You don't think that barbarian parties tried to dismantle their rule prior to that? ****, they had to mow over them in order to get where they were. Different environment, different leaders, different tools, different knowledge, different tactics, all around different circumstances, possibly a different outcome.
Even 50 years after the Oblivion crisis, Tamriel is going to be in a daze after that ordeal. They're not going to have the military wherewithal to attempt that challenge. Ionith was founded at the complete apex of their military form. And they were slaughtered. 6 whole Legions made up the expeditionary force. A small handful of survivors returned, sans Emperor. Different tools? Tamriel doesn't progress in terms of technology in a relatively quick way (arguably at all). So, same tools. Different knowledge? That handful that survived were the leftovers at Ionith, not the front of the line. What knowledge that came back was rudimentary, at best, and not significant enough to provide tactical advantage for future attempts.
And besides, where's the motivation? Going off of the point from (1), what does Akavir truly have that Tamriel does not, other than the ability for an Empire to claim sovereignty on two continents? I contest that a continent enmeshed in its own dire recovery is going to go sailing off to Akavir in hopes of conquest.
That's what I'm getting at. This kind of mystery without a payload is pointless, and is doomed to become nothing but a "could have been". At least there's a possibility that you'll enjoy the present, even if it wasn't what you expected. That's better than guaranteed fizzling out.
But the point of a present is to eventually receive it. The point of Akavir is to provide the base illusion that something is out there beyond the cardboard scenery. In the case of Akavir, the satisfaction or payload is in the mystery itself, and removing the mystery removes the satisfaction of mystery. Satisfaction of freedom of imagination over neutering said satisfaction by actually fleshing it out. Ask yourself, if Akavir is fully fleshed out, if all its depths are plumbed and laid bare, then what will then fulfill the mysterious backdrop role that Akavir currently holds?
And even if Akavir were to be fleshed out somewhere down the road, it still remains that Akavir is not ready to be fleshed out; attempting to do so now would result in a more shallow world than what it could be. As stated:
Akavir has a few demon-esque races to play with, along with some incredibly vague descriptors and a few obscure texts to its name. Tamriel currently has 5 well-fleshed-out-in-lore provinces that we haven't yet seen (plus another 1 and a half, counting the other half of High Rock, other half of Hammerfell, and mainland Morrowind). Basic and not-so-basic info has already been laid out for those provinces, meaning less effort has to be extolled to flesh them out, meaning more can be fleshed out as a whole.
Add bits and pieces of lore over a few games, build things up, and then execute Operation: Akavir.
EDIT: Displacement of quote tags; fixed.