Because some people are praying for god (or the devil as the case may be) to kill and destroy us and to show and enable them to do the same, for example.
Not all religion is benign! Not all prayer is good. It depends on the content and the direction (to whom it is oriented and for what purpose). The Church does not sanction every prayer, even every internal prayer. Some people actually worship the devil.
Some prayer is bad, and some may be at least banal. It is an sacrilege, therefore, to open up the prayer of the faithful: "and for the prayers which we hold in the silence of our hearts,...Lord hear our prayer." People might be praying for every manner of evil, in collusion with the devil. The Church cannot join her prayer and worship to that.
The Church, in her prayer, e.g. the Pater Noster, is very specific for what and to Whom she prays. She is not vague, ambiguous or sloppy in her intention or in her orientation. Her prayer does not include everything, only what is good, and right and true, and she makes that explicit, through Jesus Christ our Lord!
"We can pray with each other only if we are agreed who or what God is and if there is therefore basic agreement as to what praying is: a process of dialogue in which I talk to a God who is able to hear and take notice. To put it another way: shared prayer presumes a shared understanding of the addressee and thus likewise of the inner action directed toward him."
Truth and Tolerance, Joseph Ratzinger, San Francisco: Ignatius, 2004, 108.