Not exactly true. They both own shares on behalf of investors who are the actual beneficial owners of the shares..Google any big company "who is the largest shareholder" and Blackrock and Vanguard will be in the top 3 or 4.
example walmart WMT - Walmart Inc Shareholders - CNNMoney.com
Fact Check-Video claiming BlackRock and Vanguard ‘own all the biggest corporations in the world’ is missing context
A video claiming BlackRock and Vanguard own “all the biggest corporations in the world” is missing context. While it is true that both investment firms hold significant shares in many of the world’s leading companies and can therefore wield voting rights, shares are ultimately...
www.reuters.com