Let me guess, well, maybe the fact that we're born poor?
Poverty is the absence of a lot of things. If you're born in a small isolated tribe in the middle of the Amazon, you're poor. Nobody took anything from you, there is almost no inequality, but you're still poor.
But I think I know where you're trying to get: "They're only poor because X took whatever from them". Probably where X is capitalism, Western countries, or whoever is the fashionable enemy.
I believe Western imperialism has something to do with it... While it's true that capitalism has improved outcomes for some nations, it's also true that capitalist exploitation in Africa and South America really messed up the way their world works. It's made it so that free markets had something to fix later on, not fixed a pre-existing problem with these countries.
People can choose what to believe and all, but it's pretty hard to look at things like the African slave trade or Diamond mining in Zimbabwe or Sierra Leone and find anything but capitalism and imperialism run amok.
Look back further - while certainly not blameless they didn't start the fire - humanity in general did. In South America even the Conquistadors even with their vast advantages would have died if not for all of the other tribes sick of their flower wars and getting captured for sacrifice. They also had an army on their side. One they later betrayed and made an underclass but fifty men in an unfamiliar territory would die eventually to their might. Africa had its own warfare and ironically one of the most benign actions of trade helped set up a collapse - trade with others is how advancement is driven and they wound up benefiting from trade in more productive crops - some with precious metals and ivory and some through slaves. That lead to a population boom and that lead bloody wars of Shaka Zulu. The point being exploitation doesn't need any outside actors and is the true enemy. The true cause of poverty and backwardness is lack of growth.
This doesn't absolve the misdeeds of exploiters - indeed colonies end in independence usually specifically because of mercantilist mismanagement squanders the true potential of the country by seeing it only as a well of land resources instead of an extension to nurture for mutual good but it is important to recognize that getting out "the Imperialists" won't make things better automatically and the wrong replacement can ironically be even worse. As bad as the British were in Rwanda they never decided to genocide the Tutsis even though their manipulations lead to it indirectly. Evil comes from within and without "the tribe" and it is important to recognize that.
A common response is "Capitalism". It's an easy, glib response. Unfortunately, it's not a good one, because abject poverty predates capitalism and exists without it.
Another response is "deprivation". Unfortunately, this response is effectively an appeal to the definition of poverty.
Since you asked what I believe, I don't believe there is a well-defined single cause of poverty.
A common response is "Capitalism". It's an easy, glib response. Unfortunately, it's not a good one
If anything, Capitalism has lifted 90% of the world's population out of poverty. It's a solution to absolute poverty, not a cause. It may, however, be a cause of relative poverty. But to paraphrase Winston Churchill, it's the worst system for eliminating poverty, after all the other ones we've ever tried.
Poverty is the absence of a lot of things. If you're born in a small isolated tribe in the middle of the Amazon, you're poor. Nobody took anything from you, there is almost no inequality, but you're still poor.
But I think I know where you're trying to get: "They're only poor because X took whatever from them". Probably where X is capitalism, Western countries, or whoever is the fashionable enemy.
Nope.