Political Memes

That's my interpretation, and it's around 18-year-olds (~ my level of study in geopolitics).

Historically, Zionism is a political movement from 19th Europe century that wants to create a nation for the Jewish people as a safe haven from a long history of anti-Semitism. So it's a nationalism centered on one ethnicity and one religion. That goes against leftist ideas (named anti-imperialism in op) which are fundamentally internationalist, against religions and against ethnical privileges. Furthermore, it requires confiscating a land and replacing whatever structure was there with a Jewish one. Although, some leftist people are not foreign to this kind of action, they would do it for an internationalist ideology that they believe would improve the life of the majority, without privileging people of a specific origin.

Reality is complicated, there are many subgroups of Zionism crossed with the whole spectrum of modern politics, from far right who just want to remove all Arabic people from Palestine and replace them with Jewish people, to left people who want to negotiate a peacful two-states solution and oppose further colonization. There are probably also a lot of Israeli who were just born there, have nothing to do with Zionism, and just want to live in peace with their neighbors.