Because it's a very unstable area where terrorists (either home-grown, non-Muslim, or foreign Islamic jihadists) have controlled and/or created their own forms of government in the failed states. If nothing is done about it, the terrorism will spread to other African nations at the very least, and perhaps spread further throughout other areas of the world. Hopefully, part of the role of US-AFRICOM will be to address this issue. I would not like to see a full scale US only 100,000+ troop invasion, but I would like to see some serious COIN campaigns going on there, if they're not being done already. The African continent is rich in natural resources used globally. For that reason alone it is justified to make sure these resources aren't controlled by the wrong people. Plus all the human rights violations occuring. Perhaps we don't like having to be the world cops all the time, but I think we have a moral obligation to lead or be part of a coalition, and it wouldn't surprise me that the AU would be quite willing to supply much of the neccessary manpower.