[jowiar]

    Can anyone elucidate why having artificial barriers to employing certain types of people is beneficial? 

Beneficial to whom? The world (or humanity) as a whole? Not really. US Citizens presumably want employers in the US to employ them over, well, people who aren't US Citizens. The US Gov't is elected by US Citizens. Hence the existence of legal barriers.

    If so, why not create the same restrictions on, say, a company in California from hiring someone from New York?

Because states don't have the power to enact those restrictions. US Congress has the power "To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes.", and "The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States." See also: https://en.wikipedia.org/wiki/Corfield_v._Coryell

As far as the Xenophobia bit? Yes and no. It's very easy to argue that discrimination based on place of birth is fundamentally wrong, and that open borders are an ideal worth striving for. Not sure if anyone has figured out the pragmatic policymaking around that -- i.e. designing social structures that are resilient to open borders.