One answer I don't hear articulated often:
Because of the faulty premise widespread in the evangelical consciousness that believing the gospel at conversion sets a permanent, invariably sustained trajectory of gospel-believing for the whole of one's life.
Following conversion, do we believe the gospel, looking to Christ alone for our righteousness and joy, the rest of our lives?
Yes and no. We need to discern a distinction.
At conversion, we trust in Christ, believe the gospel, at two levels: the doctrinal level of mind-assent, and the existential/psychological level of heart-trust (what the old saints called fiducia). The snare is that we naively collapse the sustainability of the latter into that of the former. We think that because we believe the gospel doctrinally the rest of our lives, we believe the gospel psychologically the rest of our lives. But au contraire! One belief-level is static, the other dynamic.
I'm a soteriological Calvinist. At the most fundamental level, I am an irreversible 'believer' the rest of my life, by the grace of God. But at another level I move from believer to unbeliever (from faith-in-Christ-exercising to faith-in-Christ-forsaking) dozens of times, hundreds even, each day. At the doctrinal level we look to Christ with sustained, consistent permanence. But at the existiential level we keep faltering, keep swiveling away from Christ and looking to other saviors--even Christian saviors like theological erudition or Bible memory or service in the church or spiritual reputation.We can forsake heart-level gospel-trust in the very moment of defending it theologically. (Haven't you ever heard an evangelical theologian defend atonement or some related subject with self-justifying defensiveness? What's going on there?)
If we discern this distinction--if we perceive that while on one level we see the gospel in a once-and-for-all way (doctrinally) but that on another level we keep lapsing time and again into gospel blindness (existentially/psychologically)--we find one more reason the gospel is for Christians.