You're right, when you're a citizen of the world, you're a citizen of nowhere, so either nothing matters or everything does.
Back on Earth however we're British citizens, and after our immediate family and friends, our fellow Brits get first call on our concern, and if you think that's bollocks you're either out of your mind, off your trolley, or Jason from Chichester with a 2.2 degree in Media Studies from the University of Please Yourself, East Anglia.
After my friends and family, I am as concerned about global issues as I am about fellow Brits. I don't especially consider myself British, however, so perhaps that is a factor. I have no particular loyalty to my nation of birth, nor any patriotic feelings. My father is very patriotic, a Brexiteer, and told me throughout my childhood of Britain being world leaders. He spoke of British quality, and of English being the international language, discouraging me from learning others, though I was useless at languages anyway. I am now an Irish citizen.
I think you speak well of the reality for many. I don't like it, but I understand your world-view, and believe it to be largely accurate, for a lot of people. I find it somewhat depressing though, and believe that care for fellow humans would lead to better choices. I think that feelings of patriotism are dissipating fast, and there is where the tension is. Some people are fiercely patriotic, or just speak loudly about it, while others, especially swathes of young people, have no loyalty to their nation of birth whatsoever. This will be a political dilemma because young people will increasingly question the choices made by politicians, and will turn their back on political parties who support despotic regimes outed on social media. When I was young (I'm 56) I was fed tales of the evils that lurked behind the iron curtain where all was grey and could not be trusted; that Churchill was the greatest British hero; and that the British empire was filled with heroism and the spirit of exploration, and corruption was far removed from Britain but rife elsewhere in the world. This was ALL we were taught, and times are changing, rapidly. Why? Because much of that is utter bullshit, or, at best, filled with misinformation by omission.
If I were to choose citizenship of Britain, what would matter to me more than it does right now?
When I hear politicians speak of their patriotism, then watch them close mines, the textile industry, steel works, the fishing industry, and sell off social housing, their words ring hollow to me. I watch our rivers pumped full of sewage while shareholders count their dividends and patriots do nothing. I listen to Farage tell the people of Clacton that he has bought a house there, then fall apart when he's questioned about his taxes. I watch Sunak justify his 21% tax rate and his non-dom billionaire wife. I watch Starmer continue to supply weaponry to Israel while they slaughter children, doctors and journalists and I wonder how that ticks the box marked Great Britain. I watch the repeated affairs of the Royal Family, their hush money payments, their friendships with paedophiles, and I watch the press gloss over it like it doesn't matter. I suppose because I care about these issues, I must care about Britain, but I am absolutely being driven from it, both mentally, and probably physically, and I am far from alone.