Why Does the Left Oppose "Imperialism"?

I'm reading a book about a left-wing activist (A long-dead, pre-woke left-wing activist.)

The book, however, has been written recently so it uses contemporary leftist rhetoric. It repeatedly uses the word "imperialist".

I always find myself wondering why the (contemporary) left would oppose imperialism.

Is it because they think power should be devolved, that everyone should be governed by rulers who live close to them rather than rulers who live far away? Is it a sort of localism?

That sounds plausible...except the left is overwhelmingly in favour of supranational institutions like the EU, the WHO, the UN, etc. etc. Just look at the meltdown they had about Brexit.

So do the left oppose imperialism because they think people should be self-governing and not ruled by an alien government?

But what could this possibly mean according to the leftist worldview? The whole idea of an allegiance that is based on anything particularistic-- religion, language, ethnicity, folkways-- is anathema to the left. According to the left, the only permissible bond, on a societal level, is common humanity. (Even while they push identity-based grievances, this remains true.)

So how can the left possibly oppose imperialism? On what grounds? I don't get it.