It's not, the US left is represented by papers such as Jacobin, intellectuals such as Noam Chomsky, and politicians like Bernie Sanders, (who has only recently - ~last 10 years of his half a century career- joined the Democratic party), and organizations like the Nurses Union.
Sure, it's true that the Democratic Party is to the left of the Republican Party, and that there are actual leftists in the DP, such as Bernie or AOC, but the vast majority of the party is centrist at best, especially on economic issues.
On many economic issues, Hillary Clinton or Nancy Pelosi are on the right of someone like Marine LePen or Viktor Orban.
Sure, it's true that the Democratic Party is to the left of the Republican Party, and that there are actual leftists in the DP, such as Bernie or AOC, but the vast majority of the party is centrist at best, especially on economic issues.
On many economic issues, Hillary Clinton or Nancy Pelosi are on the right of someone like Marine LePen or Viktor Orban.