When people say that it's not possible to earn a billion dollars, they're talking about the discrepancy between the wealth gained by those employed by the company versus the shareholders of the company. For example, when WhatsApp was sold to Meta for $19 billion, how many of WhatsApp's 55 employees walked away with hundreds of millions of dollars?
The fundamental problem is that it's possible for an employee to generate a hundreds of millions of value for a business, and yet be compensated for a vanishingly small fraction of that. Even if the employees agreed to a particular salary, is it ethical to pay them so little in comparison to the worth they generate, or is it exploitative?
Most, if not all billionaires, reach that status by paying people far less than the value they generate. If you want to become a billionaire, you need to find people who are willing to be paid thousands or tens of thousands of times less than they're worth. You need employees who will generate you $100 million in exchange for being given $100 thousand.