Also, don't be fooled by the main business/living hubs of the glittering major cities of the world into thinking that most places are now mostly first world: Sure, you can travel from Bangkok to Delhi to Cairo and then Mexico City and in all of them spend days inside an urban area that's remarkably comparable to a normal European or U.S city's. However, these bubbles only represent a small fraction of the totality of these places, and much of that total outside of said bubbles is very, very different in how it looks and works compared to how the majority outer parts of most first world cities look or work. This without even going into any detail about difference in the countryside and smaller provincial towns that aren't specifically tourist hubs.
You are substantially under-estimating the size of the urban population of these countries if you think that the major cities only represent a "small fraction."
I partly speak from personal experience here too. The city I live in has a largish core urban zone that's fairly tourist-friendly, clean and highly, fashionably modernized and cultural (but with a charming colonial aspect).
Since the city in general has well over 15 million inhabitants, this "bubble" area itself contains well over a million people, easily, but it's just a small fraction of the rest of the city, which is very much what you'd call a dangerously third world place in terms of crime, corruption, administrative ineptitude and barely functional infrastructure.
I don't just mean 'rich countries'/'poor countries.' First, because that is not necessarily as simply true anymore - the economic relations are much more complicated. Second, that is a value judgement about how countries ought to develop.
You can read up more about it https://en.wikipedia.org/wiki/Global_South