With all due respect to sartorius, it's stuff like Homiletic and Pastoral Review that are to blame. Many clerics get lazy, and rely way too much on homily aids, to the point where, if pressed for time, they just read the homily right out of the magazine!
Many priests and deacons don't see the homily as a high priority. Until they do, they won't spend the five or six hours it takes to read, research, pray, and put together a solid ten minute homily. And, prayer is the most important part. It helps make your words part of you; they come from you, not from someone else. And they have power, when they come from you.
I can't imagine getting up in front of a congregation after an hour of preparation, or reading someone else's work. (Reading a homily is death, anyway)
But, many of my confreres regularly cheat their people of the nourishment they expect from reflection on the Sunday readings.
And, forget about doing something like Toastmasters, to improve speaking skills. They think a semester of homiletics in the seminary or six sessions of "public speaking" in diaconate training gives them all they need.
If you don't practice, you won't get better.
And, if you're not getting better, you're getting worse.
I have a good friend who critiques me every Sunday that I preach, and he's brutal. But, I need that, and so does anyone who else who thinks they know how to handle themselves in front of an audience.
It has to come from the heart, but there's nothing wrong with getting ideas or cues from dependable resources.