The argument for this is that this is the much more practical approach to foreign policy.
In realism, the US would pursue its own national interests. It would only intervene in foreign countries, for example, when intervening would make the US safer or more powerful. By contrast, idealism would have the US intervening or getting involved all over the world. The US would be constantly getting involved in conflicts as it tried to prevent governments (for example) from abusing the human rights of their people. It would constantly be spending resources trying to make other countries more democratic. This sort of idealistic policy is much less possible for the US to pursue since it would be so much more costly. Therefore, realism is more practical because it would cause the US to intervene only when doing so would help the country in tangible ways.