Sovereign nations may then enter multilateral treaties (if they so wish) to handle these matters.
Sub-jurisdictions are a fact of life.
When “city” was the top-level structure, there was more than one, and cities had conflicts (economical and wars).
Countries solve that problem, but, because there’s more than one country, introduce the problem that countries had conflicts.
Country groups (Germany, the USA, the EU, NATO, Warschau pact) solve that problem, but, because there’s more than one, introduce the problem that country groups have confocts.
The UN, in theory, solves that, and, because there’s only one, don’t introduce another, but it isn’t powerful enough.
And maybe then we'll pick up on the pattern and go straight for the United Federation of Planets.
There are approximately 30 countries on this continent. With so many actors, the chances of enduring peace over long periods of time are rather slim, no matter the external circumstances. Just look at prevalence of armed conflicts in the rest of the world.
The breakup of the Soviet Union created quite a few countries in Eastern Europe.
Also, quite a few were formed in bloody war(s) in the split up of Yugoslavia.
It is possible the UN simply isn’t structured enough to address conflicts. For example, could the recent conflict over postal rates have gone through the UN? Adding “power” to the top does not necessarily create an upper layer that leads to stability. The Roman emperor was very powerful; but the effect was a lot internal conflict and war. The balanced states we have seen in recent years — more effective than what preceded them, though in a legal sense much less powerful — are doing well at abating internal conflict in part by reducing the stakes.
Of course they shouldn't. Hierarchical structures are good and you want issues to be resolved as low in hierarchy as possible. The point of a global government is to tackle the problems that cannot be currently solved, because we're missing a hierarchy level. Because, like 'Someone pointed out in another comment, global governance is a forest, and not a tree.
The role of a global government isn't to mediate hyperlocal disputes. It's to force countries to stop letting corporations play them against each other on corporate taxes. It's to coordinate a common, simultaneous response to the challenges of climate change. It's to help everyone cooperate in solving other global challenges, where actions on one side of the world have an effect on the other side.