Because Christ knows that we do not need to know everything. We are given the knowledge, through both our own well formed reason and Holy Mother Church, that will help us in this life.
For a moment, imagine tomorrow the Church pronounced that all unbaptized children go directly to heaven. What would be the result? We would then begin to debate when a person stops being a child. Do we stick to the 7 “age of reason”? What about the mentally disabled 30 year old, do they count? Well if they count, what about my teen who has Aspergers?
Then, why baptize a infant. Let’s let them make up their own mind like the Protestants do.
On the other hand, if the official Declaration is that all unbaptized go to hell. People would despair for the children they lost, the number of illicit baptisms would skyrocket as not only every loving grandmother but some people would walk around Walmart with a bottle of water “baptizing” every child they saw.
A declaration like this would lead to more confusion, it would lessen people availing their children of the Sacraments, I cannot imagine what good it would do.