Are there any great researchers who are terrible teachers?
If you ask are there "any" then the answer is clearly yes. But, like anything else, skills are learned. I turned out to be a good teacher, beloved by my students (mostly - some exceptions). But I started out as a terrible teacher with really poor ideas about how people learn. 

If you don't do something (a lot) you aren't going to get very good at it. Being a "natural" researcher doesn't just happen either. But if you are allowed to spend all of your time on research and no one ever requires that you do any effective teaching, then you probably won't get good at it. 

Some brilliant people make too many assumptions about others. If I have deep insight into some topic, I might, falsely, assume that my students do too. In particular, I might forget how long and hard it was for me to develop that insight. 

But if your students turn out to be "just like you" then you can probably reach them ok. If you learned easily thorough lecture and you can lecture with some logic and completeness, then people like you will probably do ok. But, not all students are the same. 

Your experience of hearing "good" lectures in public settings is, a lot, due to the fact that your general educational experiences are probably a lot like those of the speaker. And if they are established and experienced, then they probably have a lot of experience getting through to  people like you. But maybe not so much in a class of beginning undergraduates.