Other's have mentioned regional stability. It's also necessary for us to protect some of the busiest trade routes in the world. If the US pulls support, the other super powers, radicals, and local scum will continue their manipulations in the vacuum we leave behind. You talk about corruption and losing a handful of forces in Niger, but how many more will suffer and how many future generations of militants will there be if we allow these groups to flourish? Then again, our presence and tactics in the Middle East seem to just create more enemies.

These are long term investments and the return isn't just monetary. Perhaps we should also consider how much money is made selling weapons and security services in unstable regions. A HUGE portion of the US and world economy relies on a steady amount conflict in the world. Top arms dealers can choose their clients and give special deals to their military friends and warlord associates around the world. I doubt peace was ever a goal in many of the ongoing conflicts.

A humanitarian side is that it's a hotbed for diseases with potential to reach pandemic proportions if not properly contained and treated. If we just abandoned all of our support and aid to the whole continent, it could very well trigger the end of human civilization.