Most large companies listed on a US stock exchange (NYSE or Nasdaq) are subject to reporting requirements with the SEC under the Exchange Act. That means they can't buy bitcoins on their own or secretly without permission or without reporting it to the SEC. Even if they silently buy BTC through OTC, they still need to report it in their quarterly financial reports. They can't hide them because it's the law.

Also, large corporations are managed by a board of directors, which means that the consent of major shareholders is required and they always want everything to be public because this involves legal risks.