The remnants of colonialism continue to produce winners and losers economically, with the winners stuck in local maxima where they extract value from the people, but the people themselves see only marginal benefit, and development is stuck at a snail's pace.
As with seemingly everything in life, the incentives for the different players really don't line up. Consumers lose, producers lose, and only a select few middlemen win anything at all.
The reason New York City is the biggest city in the US is because when the Erie Canal was built, the agricultural riches of the Midwest had a route to world markets. Where you have a major seaport, you also need major banks and major insurance companies to smooth out the financial needs of traders and shippers, providing the funds right away back to the farmers, instead of them waiting till the voyages were complete. (without the Erie Canal, New Orleans would have become the largest city in the US)
Yes, there is a lot of money in trading, banking, etc. At every step of the transaction pyramid, a %age is added to the price, and the %age fees charged on that go up accordingly. But that measures the true value of the product at each stage; if you have a cheaper way of getting the same product to the same stage cheaper, the (supposed) riches will be yours.
The socialist instinct ("anybody getting rich must be cheating") unfortunately obscures the real problem ("monopolists and cartels controlling supply and setting prices are the true enemies of the people") which hinders solving it; by putting capitalism in your gunsights, you make enemies out of natural allies.
It is not pleasant to think about it in these terms; but it does seem like some of the greatest improvements in general human welfare have their roots in relatively ungenerous undertakings by methodical, reasonable, self-interested actors. The Romans roads and the Pax Romana, and the profound legacy of Roman law, were not the result of a benevolent desire to help everyone in the world and save them from evil.
Can you provide some examples and resources to back up this hard truth?
Another way to get an intuition for it is the prevalence of various conveniencies of life. For example, I believe about two thirds of households in the world have refrigerators. About 12% of the world's households have cars. These and many other material benefits are possessed by regular people today but would have been out of reach 400 years ago to any member of the European nobility or, as Adam Smith puts it, "...an African king, the master of the lives and liberties of ten thousand naked savages.". (In his time, Smith was commenting on how remarkable it was that an English workman could have pots and pans made of metal and a window made of glass.)
But all these things were invented after colonialism of most colonial countries.
And they are prevalent in uncolonized countries in roughly the same ratios as colonized countries. Arguably the uncolonized (China being the main one) have more than the colonized (India being the main one)
It is not strictly correct to say China was uncolonized. I suppose parts of it were not.
Global trade has, indeed, brought everything everywhere; but the present operation of global trade -- the norms used, the international bodies involved, even the weights and measures -- are all the result of power projection by western, colonial powers and prudent adaptation by other countries. Even China's legal system is an adaptation of a western legal system (the civil law). The prevalence of certain institutions is something we take for granted today but does have its roots in colonialism.
While civil institutions were spread to colonies by colonialism, they were adopted by uncolonized countries also. Countries can adopt beneficial things without being taken over and having their resources and wealth extracted.
The same way Americans adopted civil rights from countries which had them earlier on. Europeans who banned slavery earlier than Americans did not need to invade America and extract its resources to bring them out of being a slave-owning polity.