But in early church history, even that practice wasn't frowned upon. The Church of England published books of homilies in the 16th century that clergy were expected to read in worship. It was a time of turbulence in the church, and leaders wanted to ensure that good theology was being taught. Some had degrees, but many had no training in theology whatsoever. But today, most pastors are expected to do their own homework.
Seminaries have classes devoted to training students to be preachers.
They not only practice their delivery, but they also work hard at developing content, spending hours studying the historical and cultural settings of biblical texts. When preparing sermons, sometimes they're advised not to look at other preachers' sermons, particularly contemporary ones. They'll short-circuit their own thinking and be tempted to shape their sermon to match the one they read. Some professors teach that it takes at least 10 hours of study to prepare a good sermon. For that reason, the Rev. Jon Lee of Dallas sets aside 7 to 9 a.m. every day for prayer and Scripture study.