Because the left sees the left/right divide as fundamentally being about opposition to/support for capitalism, with the former being the defining feature of leftism. Anyone who isn’t opposed to capitalism - like ‘left-wing’ policies that advocate for reforming or regulating capitalism instead of replacing it - are not leftist. In fact, they often act as a pressure valve for anti-capitalist sentiment within society that keeps people who might otherwise oppose it from doing so by channeling them into reformist activism instead of anti-capitalist activism. Not to mention there’s a pretty well-established historical trend of ‘left-wing’ parties appeasing or even outright enabling fascism when things get bad in capitalist economies, so it’s hard to argue that they’re even nominally left-leaning.

The US democratic party is, by the international standard (the US defines things a bit differently), a centrist party at best, and honestly probably more like center-right.