Because the "left" they let you vote for in any Western capitalist country is always incompetent and does fuck all for the general public by design. So young people naively vote for them, get their hopes up, get disappointed, and either stop bothering to vote after a few cycles (or just go through the motions of voting for whoever they despise the least knowing it doesn't do anything) or end up in the other direction on the far right complaining about the ineffectual libs (which is what they want).

They're not going to let you vote for someone who's actually anticapitalist or could actually threaten the aristocracy. The right have successfully set up strawman leftist parties that don't work and everyone hates and tries to make you think that's all the left is (at least that's my personal conspiracy theory).