Being a requirement is different than the source. It is a component and an input. You can have multiple inputs that each are necessary but not sufficient. The issue arises in describing something as the sole cause or source.

Humans need to Air to breath. Air is the source of all life. Humans need water to drink. Water is the source of all life. Humans need food to eat. Food is the source of all life.

The reality is that humans need all these things and more to live. None of them are sufficient for life by themselves.

The same is true of labor.