Happens all the time, everywhere, just not all at once. People move around every 2-3 years or so, and take their knowledge, such as it is, with them.
The latter are the ones that carry institutional knowledge as the team around them rotates. They tend to earn less but carry more influence, loyalty, and political insight. A big and indiscriminate purge/flight that includes those people can powerfully disrupt that and cause major damage to organizations and teams.
I don't know that that's happening here, but the grandparent is right to be curious.