Words become slurs as soon as assholes start using it in a derogatory fashion. It doesn’t need to make sense, it’s just how it is.

Retard used to be a medical term. Then assholes used it as an insult so it became a slur. Then it needed to change to mentally ill. Now mentally ill is started to go out of fashion.

The n word used to just be a term to refer to black people, then assholes used it as an insult. Then assholes started to just use the word “black” as an insult so we needed to switch to African-american.

Language evolves, and assholes ruin things all the time so we have to continuously evolve and change so we don’t use the same words as assholes.