Take boots, for example. He earned thirty-eight dollars a month plus allowances. A really good pair of leather boots cost fifty dollars. But an affordable pair of boots, which were sort of OK for a season or two and then leaked like hell when the cardboard gave out, cost about ten dollars. Those were the kind of boots Vimes always bought, and wore until the soles were so thin that he could tell where he was in Ankh-Morpork on a foggy night by the feel of the cobbles.
But the thing was that good boots lasted for years and years. A man who could afford fifty dollars had a pair of boots that'd still be keeping his feet dry in ten years' time, while the poor man who could only afford cheap boots would have spent a hundred dollars on boots in the same time and would still have wet feet.
This was the Captain Samuel Vimes 'Boots' theory of socioeconomic unfairness.”
― Terry Pratchett, Men at Arms
Most rich people can lay out the $50 for the good boots.
Most poor people don’t turn rich.
Most poor people can’t wait to save up $50 for the good boots; they need boots now.
This all points to the lesson that wealth and riches aren't the same thing. Rich people are rich because they have wealth, even poor people can amass riches, say by winning the lottery, but can't be called wealthy until they actually have wealth.
A person can become wealthy off the efforts of other people, but if we want wealth for all, then it's society itself that must hold the wealth. This was Karl Marx's conclusion, but all attempts to build a society capable of holding wealth for all failed.
So we're stuck with trickle-down, the idea that the surplus wealth of the wealthy is good enough for all.
Because your current system is portable and well known in a place where land use and other rights are less solid?
Because no one around you has the technical capacity to update / maintain / repair the system?
Because if you invest in something in a less developed area - someone may come and take it (yes, this includes the govt and various officials who may start talking about permitting etc etc).
Interestingly, this same question is asked and answered at the country scale for major industries. Why ship unrefined / unprocessed product that takes up 10x the volume when you could process locally for the whole country? Because the market doesn't trust the systems in that country well enough to invest $5B that has a 20 year payback even if its "easy math".
I don't feel I have enough of a grasp to summarize it. And I'm not saying it's bad or good in particular. I'm just saying, expect serious culture shock in this area of culture. You'll come away with a greater understanding of why attempted Western interventions have been less successful than expected, if nothing else.
For example, if he orders a truckload of gravel he has to watch the entrance like a hawk and be ready to rush out the instant the truck arrives, because if he doesn't the truck will just dump the gravel right in the middle of their entrance and block everybody in until he can round up people to shovel it into wheelbarrows by hand. The idea that you might want to ask where to dump the load is not something that will ever cross the truck driver's mind. The project manager won't think about it either unless my father-in-law asks him explicitly where he is planning to store the gravel he just ordered. It's these kinds of problems day in and day out that makes reconstruction so difficult.
Learning how to think ahead is something we take for granted in the west, but it's not an innate human skill. It has to be taught. Worse, if you never learned it yourself then you won't notice that your kids lack the skill. It's a difficult cycle to break.
Also, plastic bottles are surprisingly popular in Mali. Children beg tourists primarily for bottles and pens.