because steve’s argument is basically that individuals like him have a better way of resolving world problems than institutions and governments. the thing is he is an american soldier. has been in the american army. all of his friends are americans or live in america (thor excluded). so for me, an american army soldier saying that he wont respect the will of 119 nations is baffling.
my country has the right to decide who and when will enter and make havoc. steve literally went to an african country and fucked it up. every nation has the right to its own sovereignty and steve saying that the avengers, an american based group of powerful people, know better than those nations is arrogant at best and imperialistic at worse.
and this is a more historical view, but every time an american has felt the right to invade a country in the name of Freedom, that country never recovered from the chaos that they left.