There is a major reason that the United States and other similar developed countries should not engage in imperialism and colonialism. That reason is ethics. To take over other countries and cultures without their consent is immoral.
The second major issue is that with a globalized economy and numerous bilateral security agreements, attempting to conquer a foreign country might well be suicidal, sparking either major new wars or radically destabilizing the balance of power. Moreover, given the number of deadly technologies that are potentially available for retaliation, even the ability of a foreign power to safely conquer a developing nation would be in some doubt.