my question in particular is why aren't all votes considered equal in UN on any matter in general? It's so petty of US, the most powerful empire in the history of planet to put embargo on a tiny island like Cuba.
Because they are not equal. The UN is a forum where countries can communicate but it holds no power/money and relies on member states. The US pays a lot more than Equatorial Guinea abd welds more power. They US and other powerful nations sit on the security council and can veto any vote that come up there. The general assembly vote is non-binding so anyone can ignore it.
The UN was setup to prevent world war III not police the injustices
You're mixing up this vote with the security council. The US has a veto in the security council, not in the general assembly. This measure passed, but just like the measures on Russia's war in Ukraine it has no binding authority to mean anything.
