> because that's what young men want to do, dig ditches for no wage or subsistence-level wage.

It is an investment - see that bare desert over there, work off energy by digging it. Then when they "grow up" they discover that bare desert is their land to farm and their time was really a good investment in their future and gives them a good remainder of their life, and their kids a chance for an even better life.

Those who can will leave (unless they love farming), but today the majority don't have any better place to go. Stay in the village at the edge of the desert and dig in these green places means a better life than going to the city and trying to find a jobs (remember these people don't have skills in demand in the city either, other than their ability to dig ditches in the city, at least in the desert they can take ownership of the land they make better and have a farm.)

Ideally in 50 years Africa will improve conditions and industrial scale farming will take over, sending 98% of the people (there decedents) to a city where they work not as basic labor but as skilled labor (doctors, engineers...). However they are not in this position now and this looks like a step on the path to getting there.