Question

i guess it would rock becuse sandstorm is rock, but it just seems wierd that he doesnt transform...

Answer 1

It just doesn't. GameFreak probably didn't want it to be changed. Possible theories:

• Castform would need to be Rock-looking, and would be a huge contrast from the cuter versions of it in sunny, rainy, and hail weather.
• Castform would get a SDef boost if it turned Rock type in the sand, making it superior to the other forms, and GameFreak wanted to keep it balanced between the forms.