I always they don't call them "mechs" in-story because it creates the idea that nobody every thought of giant walking/battling human piloted robots before whatever crisis appeared in the story. Alternatively the in-story universe recognizes that fictional mechs exist like Gundam etc, but the new name is used to differentiate between in-story fiction and in-story fact.
"Remember when we were kids and watched mechs Voltron and Gundam on Saturday morning? Now we're Jaeger pilots!"