You may look up the individual countries you are curious about, and, in many cases, find articles concerning what the flag represents and how it was adopted.
I think a quick answer might be that most newly formed countries have some kind of tradition they are drawing on - “we used to be a nation 600 years ago before those bastards over there conquered us, and now we are again”. Often, the flag is meant to be evocative of that tradition, or a resurrection of a banner used at some earlier point in history - “600 years ago our colors were green and yellow, so now that we’re independent again, we’ll have a green and yellow striped flag”.
Monaco / Indonesia does seem to be a duplicate pattern, but the proportion is different. If someone asks you “what are the only two countries who share an identical flag?” answer “Chad and Romania”.
I think also that because flags have been meant to be easily recognized in battle, or at sea, simplicity of design became a desideratum. The importance of quick recognition has faded, but the tradition remains.
The importance of the French tricolor also should be emphasized: after the Revolution, nation after nation adopted their own stripey flags, following the French example.
Is it true about the flag for Haiti? That it was formed from a revolutionary leader tearing apart the French flag (as Haiti was under French rule), removing the white and sewing the red and blue together?