Jamieson, Fausset and Brown on Ecclesiastes 7:29

29. The "only" way of accounting for the scarcity of even comparatively upright men and women is that, whereas God made man upright, they (men) have, &c. The only account to be "found" of the origin of evil, the great mystery of theology, is that given in Holy Writ (Ge 2:1-3:24). Among man's "inventions" was the one especially referred to in Ec 7:26, the bitter fruits of which Solomon experienced, the breaking of God's primeval marriage law, joining one man to "one" woman (Mt 19:4, 5, 6). "Man" is singular, namely, Adam; "they," plural, Adam, Eve, and their posterity.