TheOtherOneEarbuds are the things that sit in the concha of your ear, and don't make a seal. For example: VE Monk+, Yuin Pk2, Apple Earpods, etc.
In Ear Monitors are the earphones that have silicone or foam tips that sit in your external ear canal, creating a seal. For example: Shure SE215, RHA MA750, these Noble X, etc.
Unfortunately, "earbuds," for a plethora of reasons, has become the term that companies use as an overarching term to describe actual earbuds (which don't have a big market in the US, or anywhere outside of China, really) AND in ear monitors.
i'm fairly certain that it is a matter of the general populace not actually knowing that there's a difference, and that "iem" or "in ear monitors/headphones" is not as easily recognized; in a word, ignorance.