Re: An interesting recap of history for those who did not live through it.
By being listed their holdings are publicaly available and there are strict and well enforced rules to prevent Samsung/Apple/Intel etc owning more than a certain percentage without announcing a formal takeover and triggering a shitload of regulatory / competition investigation.
If they were private then Dr Evil could secretly buy them and hold the word to ransom for $1Million. Which is why ARM is only viable if everybody knows who owns them