It’s called an interlocking directorate[1] and is why a lot of access to power and capital is class based and not meritocratic. It’s also part of why C level pay has skyrocketed as the members of the boards all happen to have C level positions and they all end up approving the pay increases of the people who get to vote on their pay increases

[1] https://en.m.wikipedia.org/wiki/Interlocking_directorate