Because the products of their labor are outside of their control, yet it is only through working to produce alien products through estranged labor that workers can receive the means to sustain themselves. This leads to a situation wherein the worker does not identify with his work, but only sees it as an instrumental activity to secure some end. Marx sees this as violating the integrity of man’s “species-being.” Alienation carves a line in between the life of the individual and that of species:
It turns for him the life of the species into a means of individual life. First it estranges the life of the species and individual life, and secondly it makes individual life in its abstract form the purpose of the life of the species, likewise in its abstract and estranged form.
For in the first place labour, life-activity, productive life itself, appears to man merely as a means of satisfying a need — the need to maintain physical existence. Yet the productive life is the life of the species. It is life-engendering life. The whole character of a species — its species character — is contained in the character or its life-activity; and free, conscious activity is man’s species character. Life itself appears only as a means to life. (75-6)
The life of the species is therefor said to be contained within each act of objectification:
The object of labour is, therefore, the objectification of man’s species life: for he duplicates himself not only, as in consciousness, intellectually, but also actively, in reality, and therefore he contemplates himself in a world that he has created. (76)
Because man creates not only physical objects, but also the intellectual framework he uses to hold up the ‘world’ of his ‘species life’, when his labor is estranged from him the very basis of his world, of how he conceives of himself and objects, becomes a strange power that he cannot grasp; it stands against him as a strange power that all the same dictates how he can satisfy his immediate needs.
The foundation of alienation Marx stresses is the fact that one person is forced to labor “under the dominion, the coercion and the yoke of another man” (78). This dominion is always situated in and the result of practice: it would not be the case, obviously, if workers did not labor under conditions that deprived them of their product; but Marx wants to claim further that the dominion itself is produced along with the objects workers make. The dominance of one person over another then does not stand out, as an independent fact, from their practical relations: it is made real through the way one person appropriates the other’s work, and would cease to be should the terms of their relationship shift.
Depending on how we construe the idea of “species life” this account is more, or less, plausible.