Because yes, "the left" never changes anything, and only goes further right.
(hint: That's not how this works)
Over the decades we've made massive strides in equal rights for various marginalized groups. But sometimes the dance takes a step backwards before moving forward again.
In an American vacuum I could see where you are coming from. In comparison with literally the entire rest of the world, it is clearly a flawed standpoint.
The American Democratic party is the oldest standing political party in the entire world. It last changed it's political stances in the 1960's and not because they wanted to, but because they needed to respond to the Republicans flipping the entire south in their favor.
Other countries have real leftist parties that actually get government members elected.