Simple. Because Marxists do not believe in the abstract idea of a "nation". To them, the only real country is the Proletariat class, regardless of border.
Dialectical Materialism is the cultspeak "scientific" method of Marxism. In reality, many socialists interpret that as "every person is the same". If a nigger does bad in school, it's not because of his inherent low IQ but his (insert 500 pages of college psuedoscience) oppression.
You'll also notice this with capitalists, who also want open borders to replace sovereign states and to get cheap workers. "I'm not German, Chinese, Japanese! I'm an Apple user!". Nationalism is free and inclusive (money wise), consumption is not.
In this talk from the Revolution Festival 2019, Niklas Albin Svensson (writer for In Defence of Marxism) discusses why revolutionaries must oppose any repression of migrants and refugees, and fight instead for workers of all countries to unite.
communist.red
View: https://m.youtube.com/watch?v=SmEYaFSMy6g
One of the funniest thing is that in the absolute, individualism and collectivism in capitalist/communist interpretion ends up being same.
"He's a individual, not a part of his race! Everyone is different!") (Kalergi talked about this as so called diversity of individuals replacing diversity of groups)
"He's same comrade just like all of us! We are equal!"