These are dating rules lyrics define relative dating techniques
The answer lies in the purpose of these "man-made rules." Man was made to worship God; it's in our very nature to do so.
Christians, from the beginning, have set aside Sunday, the day of Christ's Resurrection and the Holy Spirit's descent upon the Apostles, for that worship.
Sacramental grace is not something that is static; we can't say, "I've got enough now, thank you; I don't need any more." If we're not growing in grace, we're slipping. In other words, all these "man-made rules that have nothing to do with what Christ taught" actually flow from the heart of Christ's teaching.
Christ gave us the Church to teach and to guide us; she does so, in part, by telling us what we have to do in order to keep growing spiritually.
Over time, such "man-made rules" became second nature, and now we would think ourselves rude to fail to act as our parents taught us.
The Precepts of the Church and other "man-made rules" of Catholicism act in the same way: They help us to grow into the kind of men and women that Christ wants us to be.
For many of us who are parents, the answer is still self-evident.
Take, for instance, the Precepts of the Church, which cover a number of things that many people regard as man-made rules: the Sunday Duty; yearly Confession; the Easter Duty; fasting and abstinence; and supporting the Church materially (through gifts of money and/or time).
All of the Precepts of the Church are binding under pain of mortal sin, but since they seem so obviously man-made rules, how can that be true?
But what does any of this have to do with "a bunch of celibate old men wearing dresses at the Vatican"? But until a few generations ago, such an approach would have made little sense to most Christians, and not just Catholics.
Long after the Protestant Reformation tore the Church apart in ways that even the Great Schism between Eastern Orthodox and Roman Catholics had not, Christians understood that the Church (broadly speaking) is both Mother and Teacher.