There's a difference between between engaging to a crowd of colleagues and appearing engaging to a bunch of idiots.How so? In both you need to be aware of what they can process (much less for students), have a sense of their background knowledge (ditto), and be able to articular why they should care about a given topic. I actually find the tasks in general pretty similar when you think of it this way.
For the record, I get excellent teaching ratings--4.9/5 in a very demanding environment, and I do it without compromising too much on the rigor of the course. I have to compromise a little bit..a lot of my students can't add and subtract. That is not an exaggeration, and I teach in a school that is way better than the U. of Oregon. OK, now I have my credentials out of the way for our resident teaching guru, I want to say the following.
Talking about absurdly elementary concepts in front of a bunch of hung-over students who are more interested in their next networking (aka drinking) event than your class is way different from giving a talk to a bunch of academics who (1) can add and subtract and (2) are actually interested in the content of your presentation. The former is infinitely more exhausting because it feels like a performance. And it feels like a performance because very few of us are really excited about teaching very basic stuff for the nth time in a row. It was fun when I was an undergrad, but it's not fun now. Mustering enough enthusiasm about this stuff to teach well takes a lot of energy.
An actual academic talk feels engaging by nature and does not take even close to as much energy