Why did America not build its own ekranoplan?
Best answer: the United States had no need for one.   These kinds of large wing-in-ground-effect craft, while impressive, really serve no military purpose and provide no major military advantage for the US Armed Forces to possess.  

Aside from the internal Great Lakes, there are no major bodies of water within the United States of America where such a craft could be readily employed and no need to position strategic or tactical assets there.   Both the Atlantic and Pacific oceans are covered by the US Navy with the form of carrier battle groups and submarine fleets, which provide portable power projection for undersea, surface and air operations  virtually anywhere around the globe.   

Large wing-in-ground-effect aircraft do not provide a significant increase in America’s  power projection when compared with that.