Yeah, they made a big deal first with Kogane then with Mugi about how it takes decades of training to get and properly hold a human form (and even then it's annoying keeping the tail and ears hidden). So this is a fail form. For her. For me and Goro...I seem to recall it being a matter of skill level and thus something to be proud of to be able to transform all the way to the human form. Also the requirement to be allowed to visit the human world. So, likely the really furry form seems pretty half-assed, a no-talent, in their eyes, and thus not something any spirit fox would like to be seen as.