Not a physicist, but "hot" just means high average kinetic energy ("moving really fast"). So imagine a single particle moving really fast (even close to the spe

Not a physicist, but "hot" just means high average kinetic energy ("moving really fast"). So imagine a single particle moving really fast (even close to the speed of light if you want)—why that mere fact cause it to emit any kind of radiation? That would require losing energy for no reason, but from its perspective, it's already perfectly stationary. Losing energy just because you (an observer) happen to be moving with high speed in the opposite direction wouldn't really make sense. The conclusion being that blackbody radiation isn't just due to temperature, but also interactions between particles. (Which explains why it's a topic under statistical mechanics. It's an emergent property from large numbers of particles.)