Several months ago I was sitting in a Bible reading on Galatians 5 when I was struck by the realization that we were discussing Galatians 5:19–21 as though it were in 1 Corinthians. There were serious moral issues in Corinth, including incest (1 Corinthians 5:1), and the Corinthians were actually proud of it (v. 2).
But that wasn't the problem in Galatia. In Galatia, the problem was that the Christians had adopted the Mosaic Law as necessary for the Christian life. There wasn't any outright moral evil in the Galatian assemblies. So why does Galatians warn about the works of the flesh?
I think it's because Paul is warning about the end of the path the Galatians were on. They were trying to live out the Christian life in the energy and the power of the flesh, and the epistle is warning them where that would end. Certainly their motives were good, but they were walking after the flesh, and it would end up badly.