I think it’s pretty obvious why people aren’t ashamed. Because they didn’t do the act. Take your expectation of shame to it’s conclusion: It’s very selective to only call out European colonialist countries as being ones where shame should be expected. If we are looking at crimes across history everyone on the planet must end up ashamed of their ancestry as there are few innocents. Not like the current borders and genetic make up of countries in Europe has been static throughout history, let alone every other plot of land in the world.