for a broker who has more than 10 trillion in assets any stock should be easy to borrow
Maybe most of the assets are in cash accounts and not lendable?
https://www.schwab.com/legal/schwab-brokerage-account-agreement
8. Loan Consent
You agree and acknowledge that Securities and Other Property held in your Margin Account, now or in the future, can be borrowed (either separately or together with the property of others) by us (acting as principal) or by others.