It's not just the potential impact upon markets. Shareholders of that size have legal obligations as well. If you own that much of a company you are specifically and legally precluded from selling it all at once in many circumstances. Bill Gates could not sell his entire Microsoft stake all at once back in the day into an open market as it might cause the security to drop significantly and for other, legal reasons. Certainly, the securities exchange act of 1934 requires disclosures of certain sales of shareholders above a certain percentage of shares of publically traded companies, but also more nuanced and exacting laws regarding national security interest prohibit the sales of securities and ownership to specific parties or foreign interests. When the scale of your ownership stake is in the billions, that certainly limits the potential buyers of the entire stake to a short list.