Say you have a zaibatsu with 20 businesses in it. Each one owns 3% of the shares in all the others.
That means each business in the zaibatsu is 60% owned by allies. They're airtight. You CANNOT buy up enough shares to mount a hostile takeover or do "shareholder activism."
It's the business equivalent of a phalanx formation.
And for US-style capitalism, that's unforgivable.