Because wrong teaching is wrong teaching. The Church does not take a definitive stand only on some aspects of eschatology. She has on others. One of those stands was to cearly spell out that millinarianism in all its forms is false teaching.
Mind you I don't believe in the rapture but that's only because I believe that teaching is contrary to what the scriptures say about the return of Christ. That's why I can say it's wrong. If your church has no offical teaching on the events surrounding the second coming, how can it say with any certainty that XYZ theory is wrong?
Many people don't take "Because we say so" to be a valid answer either.
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