Let’s consider an analogy: the slave owner and the slave. The slave owner lives on the labor of his slaves. The slave owner takes the value of the slaves’ labor and returns a fraction of that value to the slave in the form of food, clothing, and shelter. (Suppose it would be unprofitable to let the slave die.) To your point, the slave owner can also work with his slaves in the field if they so choose. And maybe the slave owner has a pleasant demeanor and treats the slaves (relatively) well. But no matter how he works in the fields and no matter how nice he is, the slave owner is still living off of the value of the slaves. Moreover, in a system of widespread slavery, he needs slave-labor in order to compete with other slave owners.

You may object that there are several disanalogies here. The modern-day worker can choose who gets the value of their labor. The experienced worker can negotiate higher wages based on higher earning potential. The successful worker may acquire enough money to go into business for themselves and hire others.

Perhaps these are fair objections but they do not touch on the point of the analogy. There is a fundamental distinction between the slave owner, who lives on the labor of slaves, and the slaves who labor for the slave owner. Similarly, there is a fundamental distinction between the capitalist, who lives on the labor of workers, and the workers who labor for the capitalist.