You are leaving out the most important thing: capitalism. Imperialism, like its predecessor colonialism, is driven by the capitalist need to expand forever, control every resource.

You know who said it very well:

The discovery of gold and silver in America, the extirpation, enslavement and entombment in mines of the aboriginal population, the beginning of the conquest and looting of the East Indies, the turning of Africa into a warren for the commercial hunting of black-skins, signalised the rosy dawn of the era of capitalist production. These idyllic proceedings are the chief momenta of the era of capitalist accumulation.

The values of white supremacy play an important role here, of course, but they are not the primary cause.