Unions span across corporations within business sectors whereas corporate boards do not.

What about Blackrock? Their representatives sit on many boards in a single industry. Same with large VCs. It's not as structured as unions, but each board is not a completely independent entity either.

> What about Blackrock?

This is actually a good point, and is detrimental for similar reasons: concentration of power, reduction of competition and increased fragility of the whole system ("too big to fail").

Actually, BlackRock is incentivized to have the whole system be healthy.

Business associations do however.

Yes, board members are strictly segmented by business sector, by law. I mean, god forbid we could have someone on the board on an aerospace company, a car manufacturer, and an internet behemoth! /s

Well, i have something you might want to hear about. It's called investment funds and weirdly competition between companies started to fall as investment fund rose up. They also are present at a lot of different director board or committees.

I've never felt pressured to join a corporate board against my will.

Have you ever felt pressured to do work for a corporation when you'd prefer not to? (E.g. take a job for the health care/other life needs).