The Catholic Church isn’t so much interested in inches as it is in giving a standard that clarifies the boundaries of what is modest and what is immodest dress. Without this from the Catholic Church, we would be on our own trying to figure out a battle that makes no sense without clear boundaries and a common sense foundation that we know is strictly from God, and helpful in situations.
Without a modesty standard we run amok trying find what feels modest, what looks modest to us. The problem with this is that, one, the Catholic Church is the Bride of Christ and as Catholics we can trust that she never errs. Trusting in the Church casts off all doubts and worries about validity and “what ifs” because we know the Church has already thoroughly gone through the issue and has made its official stamp on it.
A second problem is that not all people are tempted by the same things; a mini skirt may tempt 90% of men (and women) but 10% may not. Does this mean we should go ahead and wear mini skirts? Or not? Why even go to what tempts men and women? With an official standard from the Church we know what IS modest, and approved by the Bride of Christ as modest in dress.
All in all we are called to dress modestly not JUST because it tempts others, but because we love God and we do not want to offend Him by sinning. We want to be pure and holy and get to heaven when we die.