If you had a world wide petition, you'd have 1 of 2 things: 1) everyone participating in one giant binding petition, in which case you're already imposing world

If you had a world wide petition, you'd have 1 of 2 things: 1) everyone participating in one giant binding petition, in which case you're already imposing worldwide democracy on them without asking them (the act of making them vote on it in the first place is an undemocratic act), or 2) individual nations and groups being given the option to have a vote on the matter, in which case you'll certainly be left with nation states that say "no". People don't want what you're proposing. Is it not then undemocratic to impose it on them in the first place?