I read this earlier, but have not yet responded. It deserves an answer. Too often we say “these are the rules” without explaining the “why” of things.
The reason is that Confession (specifically absolution) reconciles us to both God and the Church. Think about the “and the Church” part. In order to be reconciles to anything, that is, in order to be brought back into good standing with any organization, community, or even person, one must first be united to the other. If I’m a member of the Raccoon Lodge, but fail to attend mandatory meetings, I’m no longer a member in good standing. In order to be returned to full status, I have to make good what was lacking before. I can have my membership re-instated and be brought back to full membership. But realize this: I cannot be restored to membership unless I was a member in the first place.
Sin separates us from the Church, to a greater or lesser degree depending upon the severity of the sin. A very minor sin separates us, but only a little. A very serious sin separates us more so. Absolution then restores us to full Communion (as much or as little as is needed).
Since non-Catholics are not first full members of the Church, they cannot be restored to a state which they never had in the first place. Only after being received into the Church as full members can they be in need of restoration. That’s the reason why absolution is only possible for Catholics.
I hope that can address your concerns.
Now for some rambling thoughts…
The already-baptized go to Confession as one of the final stages before being received into the Church. This is a necessary exception. Frankly, there is a contradiction between the theology of absolution (what I wrote above) and the necessity of being absolved before First Communion and Confirmation.
The actual way that RCIA for the already baptized was designed (and this is still the Church’s current norm of law) so that a person would be received into the Church but NOT at the Easter Vigil Mass. This is a practice that developed in recent years—to combine the non-baptized with the previously-baptized. Once we started that, it opened new problems. The previously-baptized should actually FIRST be received into the Church, THEN go to first Confession THEN receive First Communion and Confirmation. Most people don’t realize it, but the practice of receiving adults into the Church is still reserved to the bishop, and only by special delegation from the bishop may pastors receive them. The way we often see it is not the intended way, it’s only done by special permission.