Because it is our natural tendency to do so. It is our natural tendency to add works to grace as part of the way we are accepted by God because grace, being utterly free and unconditional to us sinners, seem so unlikely, so unreasonable, even impossible. We also tend to corrupt the gospel of righteousness with God by grace alone, through faith (resting and receiving) alone because we are “hardwired” (as Mike Horton puts it) by creation for the covenant of works. Mike didn’t invent this way of thinking. David Dickson said the same thing in the 17th century:
Unto this Error of seeking Righteousness by our Works, after entering in the way of Justification by Grace, we are all naturally inclined; for, the Covenant of Works is so engraven in all Adam’s Children, Do this and live, that hardly can we renounce this way of Justification, and howsoever it be impossible to attain Righteousness this way, yet hardly can we submit our selves to the Righteousness by Faith in Christ, which not only the Expereince of Israel after the Flesh maketh manifest, but also the Experience of the Galatians lets us see; for, they having once outwardly renounced Justification by Works, and embraced the Covenant of gracious Reconciliation by Faith in Jesus, did turn about for a time, to seek Justification by the Works of the Law, and were on the way of falling from Grace and Communion…
David Dickson, Therapeutica Sacra (Edinburgh: Evan Tyler, 1664), 298.
Read more of and about Dickson at James Durham Thesis.