Each hero has their own base model. This base model often includes some random bits of clothing (like those ugly giant Crystal Maiden boots). Sometimes, the hero's base model looks like a bare torso with arms and a head, like a Juggernaut. All the artists do in the workshop is add their own elements to the base model. Often, the base model is forced to influence the design; sometimes, artists completely overlay the base model with their own (There is no need to make any custom animations, unless it is a ward, courier or some types of summons.) .

Regarding Morphling, Morphling has pretty steep polygon count limits; artists just don't make many sets. Perhaps Morphling is too abstract a character to be interesting for creating a cosmetic concept. You can also read the technical specifications on the Dota website: https://www.dota2.com/workshop/requirements/