Lorewise, Alliance and Horde have fought side by side for many expansions and the leaders from both have good relationship, and the concept of them being these archnemesis seem to vain out.
Why dont they ally them self, are they politically/ culturually different or what are the reasoning?
Afaik the factions only exist for gameplay purposes, story wise it makes no sense anymore. Blizzard usually wants us to remember the hatred between the two factions by picking a scapegoat (usually from the Horde side) who causes a bit of drama and then with the power of friendship and togetherness we fight as one.
So in short: it exists only for gameplay purposes.
Lore-wise, the Horde and Alliance are two distinct sovereign nations. They are politically and culturally separate. They are their own entities and have their own betterment/continuation in mind.
It’s like any IRL medieval kingdom when dealing with their neighbours. Neither wants to submit to the other due to differences, but they may band together to defeat a greater threat.
When a massive threat looms over Azeroth, the Horde and Alliance become allies of convience. They now have the same enemy and a common goal. That doesn’t stop them wanting to keep their own ways and remain separate. There’s also the shedload of past animosities that will keep the common folk opinionated against their rival Faction.
You could also take the West and the Soviets banding together against the Axis in WW2. They didn’t suddenly become one massive allied nation - even among the US, UK, France, etc, there was no sudden merging.
The US and the Soviets had massively opposing ideologies but they both came together against a common foe, then went their separate ways… into a different kind of war.
Seriously hope they don’t do a BfA 2.0 . The two have collaborated together countless times to deal with bigger threats, at this point it’s nothing but forced…
Sure, have some splinter group within the factions causing trouble… but not a central focal point. PvP-wise, faction fighting will remain for the sake of gameplay. Storywise, no.