Because they go together.

The hallmark of developed countries is that they're even, mostly egalitarian and developed everywhere.

The hallmark of developing or underdeveloped countries is precisely the staggering levels of inequality.

Not everyone is poor in a developing/underdeveloped country. Quite a few people there live lives that would make upper middle classes in developed countries blush. Life "just" sucks for the majority of people there.

> The hallmark of developed countries is that they're even, mostly egalitarian and developed everywhere.

Nope:

https://en.wikipedia.org/wiki/List_of_sovereign_states_by_we...

Developed-but-very-unequal, and less-developed-but-more-equal are a thing.

Correlation is not causation.

Would you say USA is a developed country, considering for example Los Angeles slums? And there's a bigger inequality between Elon musk and a well paid software developer than between a poor person in a developing country and a rich politician from that country.