I'm asking this because I've attended many lectures given by well-known researchers and somehow they're all surprisingly good.
While I can't identify any academics that are good researchers and poor teachers, I will note that there is good reason to think that good research and good teaching would be (at least weakly) positively correlated, and therefore it should not be surprising that most well-known researchers are also good teachers. Moreover, there is a self-selection effect in academia, whereby the most highly successful researchers often have the ability to negotiate away some or all of their teaching duties, and so those that remain as teachers tend to be the ones who are good at it (at least by self-perception). (Note that there is quite a bit of meta-analysis on this subject in the education literature; see e.g., Hattie and Marsh 1996; Zaman 2004; Elken and Wollsheid 2016.)
In my experience, the main three elements that good teaching requires are: (1) having a comprehensive understanding of a subject from first principles; (2) the ability to be able to build the material up slowly and patiently, with respect for audience context and existing knowledge; and (3) the ability to anticipate and address common areas of confusion, misunderstanding, and other pitfalls. Most of the best researchers have at least the first of these three traits, which already gives them the baseline requirement to be excellent teachers. Also, most well-known researchers are highly experienced academics, so they have probably also taught a lot of courses, and have built up a good knowledge of how to go about explaining the basics, and anticipating the common confusions, misunderstandings and pitfalls.
In theory, it is possible that becoming an expert researcher on the "cutting edge" of a field could harm one's ability to be able to remember the common confusions that one suffered from as a novice, or harm one's patience in dealing with these confusions. However, in practice, most academics do enough teaching from time-to-time that they maintain their ability to slowly and patiently explain material, and anticipate and address misunderstandings. For those who do not have the patience and inclination to explain the basics in their subject, if they are successful academic researchers, then they have probably negotiated their way out of teaching duties, and thereby self-selected out of the teaching pool.