Probably and possibly are not good enough. They want to be as sure as they can that a growth is not malignant, and if it is, that all the cancer has been killed. I would submit that in the case of eternal salvation, the stakes are even higher. Moreover, the uncertainties of speculation about a broader scope of salvation than that attested to in scripture are even greater than those having to do with cancer outcomes. So we should be very careful and err on the side of what is sure and what we know to have been revealed in scripture.
Pastors are physicians of souls.
Christians are called to be a royal priesthood to a dying world. We should be as sure as we can that people have passed from the death of this world to the eternal life which comes in trusting Christ. We know that Jesus Christ promises that whoever comes to him, he will in no wise cast out. So if we proclaim Jesus Christ to our flocks and to the ends of the earth, it is sure that whoever trusts him has eternal life. The alternative speculation, that God will save others too, while I cannot disprove it and have no desire to, is not really supported by the Bible and is quite possibly if not quite probably, false.