The concept of a 'social contract' comes from Jean Jaques Rousseau, one of the central 18th century Liberal philosophers. 18th century philosophy did not always (or even often) make clear distinctions between descriptive and prescriptive modes — philosophy hadn't yet begun to grapple with the is/ought distinction — so the point of this concept was both to explain the observation that people naturally and willingly form communities that limit the behaviors of individual members, and to rationalize that natural behavior into an abstract, rational construct.
The idea of an unbound Liberal individual, by contrast, comes from John Locke's theories, but even Locke held that where these isolated individuals come into contact with each other they establish contract-like behavior, respecting certain boundaries (e.g., not violating each others natural rights) and creating long-term or pro tem cooperative agreements. No Liberal theorist that I know of ever theorized that men prefer to live in complete isolation from each other, eschewing the benefits of society, and every Liberal theorist who discusses social life does so in terms of rational, contract-like behavior.
Remember, a social contract not only protects the weak from the strong, but also protects the strong from mass action of the weak. Many tyrants who break the bonds of the social contract with their citizens find themselves in the wrong end of a noose at the hands of a mob.
Whether or not one likes the concept of a social contract, one ought to recognize that jails are filled with those who refuse to buy into it. The social contract is a fact, and one must either abide by it or work to change its tenets.
