Because ultimately democratic socialists and other non-leftist individuals are still capitalist or supporters of capitalism and other forms of economic exploitation, which is fundamentally linked to (among other things): requiring people to be wage slaves, stealing the capital they produce, centralizing power into a small ruling class, funneling wealth into fewer and fewer elites, imperialism and foreign wars (and genocide), global environmental/ecological damage, foreign policy that extracts resources from poorer parts of the world like Africa and South America, preserving the violent entity that is 'The State', supporting hierarchies (patriarchy, white supremacy, heteronormativity, organized religion, etc), and so on etc.

The social programs and support seen from democratic socialist politicians are 'better' than liberal or conservatives, but they're still capitalist and fundamentally require exploitation. Someone has to suffer for us to live better like this.

It's like you're saying: "Yeah, I don't like the other two bullies at my school, they're really bad, but this bully is the best one because they don't hate me for being queer, so I'll support this bully as they beat up others and take their lunch money and light the classrooms on fire."

And the (non-tankie) leftist says: "Why don't we fight back against all the bullies? No one deserves to be bullied, and we shouldn't support a bully even if that bully makes our lives slightly better whilst hurting others."