Without exception, each of the epistles were written to people who were already "saved." The epistles were written to groups of saved people from Rome, from Corinth, from Galatia, and so–on. In addition, some of the epistles were written to individual saved people, such as Timothy, Titus, or Philemon.
When you were young, your mother never told you how to become alive...she only told you how to live, and how to remain alive. She told you such things as "Eat your vegtables," "Tie your shoes," "Brush your teeth," "Put your coat on," ...and things like that. She never told you how to become alive, because you were already alive.
Likewise in the EPISTLES, the apostles didn't tell anyone how to become saved; they told them how to live, and how to remain saved. Why tell someone that they need to be born again, when they have already been born again?
Each of the cities for which the various epistles were named had a local congregation to tell the unbelievers how to become saved. This was done face–to–face, or speaker–to–group, just as recorded in the book of Acts. In the epistles themselves, however, the apostles wrote to the various people and congregations in order to instruct, and to encourage them to remain in the faith.
It really is that simple.
However, on occasion, when it is relevant to whatever particular topic that the apostle is writing about, that particular church's members having become saved (in the past tense) is mentioned in the epistle. And so, water baptism is mentioned, even in the epistles.