Nothing prevents the major companies mentioned in OP from accumulating Bitcoin, and they may be accumulating it now without announcing it publicly.
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.
Thank you for the explanation that clarifies the decision-making process of these giant companies. I can also see your explanation answers the question at hand: why these companies have not yet joined the Bitcoin accumulation race, in which countries are now competing, as are smaller companies that have developed and are working on a plan to accumulate as much Bitcoin as possible. Although these companies are private investments, decisions are subject to a somewhat decentralized system, with decision-making being distributed across several boards and subject to strict legal regulations.