You call nationalism a cult, I call it a grand narrative underpinned by shared values that allow people to more easily trust each other (and hence coexist).
It's the same thing at different levels of abstraction.
If you are from a locale where low-trust, feuding groups are interacting with each other but on a largely negative basis, and you're tired of it - then nationalism looks like something positive that can unify people into something more peaceful, and more capable of doing large scale projects together etc.
If you are from a locale where low-trust feuding nations are interacting with each other on a largely negative basis, and you're tired of it - then you want to develop a global structure which unifies them into something more peaceful, and more capable of doing large scale projects together etc.
What is it about nationalism that makes that particular level of abstraction become reified and hard to move beyond?
Pride I suspect is a large difference. While it might be accepted to alliance of neighbors foreign "others" don't cut it unless conquered as it doesn't vicariously massage their ego the same way to have partners with agency.
It seems to go back to before nationalism per se as a term. See Genhis Khan and the Mongols who ironically had little cultural influence over those they once conquered. Just uniting into a lack of war wasn't enough - they needed outside targets. Feudalism in general had that issue where a post unified country is left with an excess of fighters to burn off on foreign adventures for stability to prevent infighting fiefdom expansions. Both Japan post Sengoku Jidai and the Crusades fit that model as well. It also made them long term enemies even jf it didn't result in same generation "show up to pillage your holdings" retaliation.
You can't abstract around how our brains are wired and they are wired in a way that wants to define ingroups and outgroups. Unification is a futile concept. Humans will just figure out how to create ingroups and outgroups based on a different basis. Maybe they'll use clothes branding, like back in high school. It would be better to go with decentralisation and fragmentation, so you have lots of small groups, but everyone from that small group would be part of the ingroup.
Pride I suspect is a large difference. While it might be accepted to alliance of neighbors foreign "others" don't cut it unless conquered as it doesn't vicariously massage their ego the same way to have partners with agency.
People seem to prefer something which flatters their ego over something that works.
That just isn't going to happen globally.