Why isn't the Rapture specifically mentioned in Revelation?
The book of Revelation implies a Pre-Tribulation Rapture without specifically mentioning the event. In chapter 4 verse 1, John sees a door open in the heavens and he is raptured to Heaven in what appears to be a symbolic type of the Rapture of the Church. In Revelation 19:11, John sees the heavens opened again, and Jesus descends on a white horse, with the Church accompanying Him (Revelation 19:14). Chapters 2 and 3 of Revelation focus on the Church, but there is no more mention of the Church until the end of the book (Revelation 22:16). From chapter 4 through chapter 21, there is only mention of "saints," which would be those who accept the Gospel during the Tribulation.
The Bible never specifically defines the timing of the Rapture by tying it to any other event like the re-establishment of Israel or the rebuilding of the Temple. Even its proximity to the Tribulation is inferred rather than definitely stated. That's because the Rapture is an imminent event that could occur at any moment.
There are strong scriptural inferences that the Rapture will occur before the Tribulation begins. A couple of those are found in Jesus' Olivet Discourse as recorded in Luke 21. Jesus stated that when the end time signs "begin to take place," we are to look up, for our redemption will be drawing near (Luke 21:28). Note that the redemption will come not at the end of the signs, but as they begin to take place. Jesus then added that believers should pray earnestly that they might "escape" the Great Tribulation that is coming upon "all those who dwell on the face of the earth" (Luke 21:36).
In this regard, Paul states in 1 Thessalonians 1:10 that believers are waiting for "His Son from heaven... that is, Jesus, who delivers us from the wrath to come." And Jesus told John to write to the church at Philadelphia, representative of faithful Christians, that "because you have kept the word of My perseverance, I also will keep you from the hour of testing, that hour which is about to come upon the whole world, to test those who dwell upon the earth" (Revelation 3:10).