So my brother is a big time leftist and we've talked about this stuff a lot. I'm gonna attempt to explain where I think leftists are coming from, or at least how my brother sees it:
To leftists, centrists are the biggest road block when it comes to progress because they're the keepers of the status quo. A lot of leftists believe in accelerationism, so, they believe, if right wingers got their way things would change so drastically that people would be up in arms and fight for progress. As it is, things are just good enough for a number of people to keep the populace pacified, but still terrible for a huge number of people. This system is propped up by moderates and centrists and "neolibs". At best, these people are for incremental changes but these changes are so small as to be negligible; add to that increasing wealth disparity and climate change and it's clear things need to change quickly. So moderates, to leftists, are at best cowards too afraid to embrace progress or, at worst, terrible people that refuse to see how bad things are for their fellow countrymen because they personally don't have it so rough.
Not a great summary of leftist thought, not to mention it paints a pretty big brush, but this is what I've gleaned from my discussions with my brother.