The reasons for this lower level of productivity are many. Additionally, the reasons vary from one unproductive individual to another. Some individuals are unproductive, in terms of the value they add to the items upon which they work, because they are less intellectually gifted. This is certainly not their fault. Some individuals are unproductive because they are physically unable to work.
Some individuals are less productive for other reasons which are of their own doing.
Some individuals choose not to work. Some individuals choose not to educate themselves. Some individuals are simply lazy. Of course, some individuals enjoy a combination of these attributes. Socialism, in its effort to not only believe that all individuals are equal, but to put this belief to practice, appropriates the resources of the productive for the benefit of the unproductive.