Janet Yellen isn't a powerful person because of her reputation on the speaking tour circuit. If you hire Beyonce it's because you're paying for the performance she gives. If you hire Janet Yellen, no person credibly believes it's because that hour talk she's giving is hugely valuable. It's because you want to establish connections with her to influence her in her actual job where her power comes from. It's a pretext for paying her money that, were you to turn up to her office and hand it to her at her day job, would be a bribe.
From my own personal experiences, the most you typically get with a high profile speaker is the email address of their assistant or the agent from the speakers bureau.
You can watch performances on Youtube as much as you like, with perfect sound. Most of the time, you hire in-person Beoynce for prestige. Same with Yellen.
