The unmentioned scandal of late-20th/early-21st century preaching is not the number of preachers seeking outside inspiration for their sermons. That's well-documented. Just as baleful in its effect on modern preaching has been the number of preachers who have turned from preaching sermons to a flock to writing sermons for an audience.
My father used to tell his children that written and spoken English are two entirely different languages. And it's true. Equally distinct are sermons addressed to a particular flock and sermons written for a broad reading audience.
Throughout the centuries a number of great preachers have had their sermons collected and published. But the sermons contained in such volumes were seldom prepared with publication in mind. Rather, they were preached to a congregation (often from notes or extemporaneously) and later made available for publication. For instance, John Calvin's sermons are available in written form due to the work of a French shorthand expert named Denis Raguenier who took verbatim shorthand notes from Calvin's extemporaneous preaching. Luther's sermons were carefully recorded by a variety of listeners. Martyn Lloyd-Jones' sermons were mechanically recorded and later transcribed. Jonathan Edwards sermons are still being culled from handwritten manuscripts and must be edited for publication.
Today many preachers write sermons with minds divided between a local congregation and the national audience they hope to reach through a book they intend to publish out of their current sermon series. The result is wretched preaching (and often wretched publishing to boot).
Many of the greatest preachers of our time have declined in power as they have shifted focus from a local church and specific congregation to a national audience. If you listen to early sermons by many preachers whose sermons are routinely published today you find power not present in current preaching. Sadly, the powerful preaching of a young pastor often leads a publisher to offer book contracts for future sermons and those book contracts become the death of the preacher's power.
It's also true that preachers whose sermons are routinely broadcast often fall into a similar trap of preaching for a broad audience rather than preaching to build a particular Church
Preaching for posterity rather than for the current needs of a particular flock leads to emasculated preaching. Those who preach for publication can be divided into two camps: populists and scholars. Populists tend to become more oratorical, to illustrate more liberally and to over-simplify. Those preaching for more scholarly publication (commentaries, for instance) become more pedantic and theoretical. Both types of preaching are devoid of pointed application.
Many good preachers also write great books. But in the end, I am convinced that preaching for publication is preaching for pay and honor. And that always bodes ill for power.