Ask Lemmygrad

Western marxists feel personally attacked for this position and end up rejecting it and discrediting it.

This is certainly part of it, but there are at least three other reasons western marxists hold some reservations:

1. Inside the imperial core, it's often framed as essentially a defeatist position. If a leftist from the U.S. accepts the idea that pretty much everyone around them is inherently reactionary, what are they supposed to do? You can't decide at the start that there's no way to win.
2. While the material conditions of a poor person in the imperial core are better than poor people in the imperial periphery, the imperial machine rarely ties its exploitation directly to that benefit. A key part of modern imperialism (especially in the U.S.) is denying that you're an empire at all. When that's combined with obscene inequality in the core, you have the basics for building class consciousness even if on paper your imperial working class is better off than working people in the rest of the world.
3. It occasionally veers into determinist/essentialist arguments, which have all sorts of problems.