Objectivism doesn't start at rational egoism. Rational egoism is epiphenomenal, a natural outcome (or perhaps symptom) of deeper elements of the objectivist worldview. At heart, objectivism constructs a heroic perspective of humankind, built on the following principles:
- That human existence — the existence of the conscious self — is the objective grounding on which everything else rests
- That this conscious existence depends on the rational and systematic application of thought to the (objective) material conditions in which it finds itself
- That (finally) the fundamental nature of human consciousness is to secure the conditions of its own existence.
Rand's heroes — as we see in all her books — are those who walk a narrow line of reason. On one hand, they do not give deference to social authority or established norms, which objectivism views as unconscious (unthinking) reactivity. On the other hand, they do not engage in frivolous or fanciful indulgences, which are the flip-side of that social deference: an unconscious (unthinking) embracing of social norms and values. Reason and will become the sword and shield by which one defends against unconscious dissipation and cuts a path towards existential security. But they are weapons one must pick up and use; one must fight against a natural human tendency to drop them in favor of easy intuitions, reactions, and urges.
It's worth noting that Rand (in context) was responding to an implicit nihilism that was common to the philosophy of her day. Much of early 20th century philosophy was built on Nietzsche's rejection of conventional morality as hollow and corrupt formalities. However, while more notable philosophies like existentialism, phenomenology, and absurdism focused on the (essentially negative) task of breaking down the hold that social constructions and conventions have over us, objectivism merely brushed that problem away, blithely differentiating between those who manage the task through will and reason and those who do not. In a sense, Rand created a kind of secular Calvinism, in which material success becomes the measure of the inner (spiritual, though Rand would despise the term) success of reason.
While it may be possible to rectify objectivism with certain forms of Marxist thought, as a general rule collective action naturally subsumes the will and thought of the individual to a form of collective authority. One can imagine, perhaps, a union forming because every single worker independently decides it is in their best interests, but in reality such things usually become a matter of loyalty: to our fellow workers, or to the company that employs us; to the present subsistence granted by a paycheck, or to a brighter future... For Rand, this is merely a contest of unconscious reactions, where we pit our fear of unemployment and poverty against the fear of social stigma, and never have the opportunity to stop and think proactively about what is best for ourselves.
However, Rand's implicit secular-Calvinist tendencies create a bias. Those who have materially succeeded — business owners, corporate CEOs, etc — are presumed to be rational by virtue of their success, and thus are taken as examples of the objectivist ideal. Those who are consigned to the workforce — who live on the edge of existential failure because they depend on a weekly paycheck from others — have failed the objectivist ideal, and are presumed to be irrational. The collective activity of irrational actors is ipso facto irrational, and thus union formation, social-interest politics, or any of the other things that are often loosely lumped under the label 'socialist' are necessarily anti-objectivist.
In the objectivist world, the problems ostensibly created by capitalism are actually created because laborers don't exercise the power of thought and reason. Rand would argue, I imagine, that the Marxist concept of exploitation only has reality because laborers allow themselves to be exploited. If laborers used reason they would all individually decide not to work for less than they wanted. If that meant that they starved or became homeless while struggling for their desires, so be it. Such struggles are part of the heroic journey of being a human consciousness, and even a heroic death-in-misery is better than living in thrall to irrational dictates. As a result, Rand holds capitalism to be faultless: success or failure is a measure of the individual's capacity to rational action, and no societal or contextual factors matter, because those are merely obstacles to be overcome by reason and will.