I find that a lot of people just like to feel superior to others, and slinging ableist slurs is an easy way for them to do that because it puts the other person down without them having to actually invest any critical thought in to why.
They also generally already look down at disabled people, so they don't see anything wrong about using these slurs since the people they think they apply to, are, in their minds, already not worth considering as part of society.
You're absolutely right about the similar arguments that extent to other bigotry and toxicity, especially in leftist spaces where you'd hope people would know better. I think this article also does a great job at explaining how the words we use shape the spaces we occupy, and who does and doesn't feel welcome and included in them., but unfortunately as with so much other information, the people who need to hear it most are those who refuse to listen..