As retrac says, it’s easier to cast for humanoids. And decades ago the special effects technology was nowhere near what we have now, so there were fewer options

As retrac says, it’s easier to cast for humanoids. And decades ago the special effects technology was nowhere near what we have now, so there were fewer options for more unusual forms and they were expensive. Novelists had no such restrictions, so Larry Niven could create Pierson’s Puppeteers in 1966: https://en.wikipedia.org/wiki/Pierson%27s_Puppeteers https://en.wikipedia.org/wiki/Pierson%27s_Puppeteers