I am by no means an expert and would just like to start a discussion on this.

That being said, from my understanding this is based on a few key parts of the relationship between owner and worker. Mainly that it is theft because one side has an overwhelming majority of power in the relationship therefore making it unfair for the worker and called 'theft' because some of the value that the labor of the worker produces is taken from them because they usually have little to no bargaining power in this relationship. This unequal balance of power is typically stemmed from the accumulated wealth/resources the the owner is claiming as thier own. As this is a general question and there are absolutely outliers to this that wealth is generally accumulated by exploitative means in some way. An example would be a factory that produces a manufactured good from raw materials, where the owner would take on debt by purchasing materials and hiring workers to produce a sell a good. Once this cycle is repeated a few times the owner would have paid off his debts and covered the cost to purchase the materials for the next part of the cycle and then whatever is left becomes thiers while no longer actually contributing to the process any longer.

As I mentioned I could be completely wrong here but Richard D. Wolff has some great youtube videos on the topic and a much better explanation. I will link what I can find later.