You are neglecting the Orthodox and Coptics who are not Romans but believe in the necessity of confession and absolution for the forgiveness of mortal sins as well. The idea of a practicing Christian not confessing and seeking absolution is a fairly modern innovation held by only a small fraction of the world’s Christians, the slothful not counted.
I don’t recall the Bible saying anywhere “go tell God in secret about what you think your sins are and all will be forgiven.” On the contrary the Bible gives the apostles, and by extension their successors, the power to bind and to loose.
As to the practice, lots of people give up on the Christian faith, to quote Chesterton, “not because it was tried and found wanting but because it was found difficult and not tried.” Plenty of Protestants fall into the practical Atheist and Agnostic categories as well. Telling another person about your wrong doings isn’t comfortable or easy but a good confessor can help to get to root of the sins and you recognize other sins by pointing out the blind spots we all have when looking at ourselves.