Some of these people are investors or represent investor groups. As such, the group they represent will own stakes in multiple companies, and often have board seats on many of the companies.

Others will be an top executive at one company, and also serve on the board as an advisor of a few other, non-competing companies (usually non-competing).

Those are probably the most common scenarios where some one would serve on multiple boards.

They Rule is outdated, but the unfortunate truth is that there's been even more consolidation since it first went online. This is partly due to the consolidation of liquidity through institutional investors. It's one of the consequences of passive investment strategies being dominant among retail investors.

It’s a big club, and you ain’t in it! https://m.youtube.com/watch?v=Nyvxt1svxso

Companies, state dept., politicians, revolving door