I don't think this should be surprising. The celebrities that people would want are rich enough that they don't need to say happy birthday to your friends. The celebrities using Cameo are the ones who are broke, or don't want to do a toothpaste commercial, or who can't get work because they're politically extreme, or simply not popular enough to keep a steady stream of (insert whatever they usually do) going.

Cameo stops being a good idea when the people doing the videos aren't exciting for the people who want the videos. We crossed that line years ago.