I've checked the Stud.io folders and while I could indeed find a few parts that don't appear in the actual program, I don't know why they put them there.
Regarding the Duplo parts you mentioned, I had no issue importing them. I didn't even 'need' to implement connection points - for simple builds the collision + grid does a well enough job:
You mentioned that you used these parts some time ago - I guess this was prior to at least one Stud.io update? Have you tried to clear your custom parts folder and try import them again?
Regarding Lego's stand on Duplo parts: I really don't think it's a technical issue, given the relative simplicity of (regular) Duplo parts. Since they didn't bother adding any type of Duplo bricks to LDD back in the day, I believe there is a different reason for Lego not to include them.
Regarding the Duplo parts you mentioned, I had no issue importing them. I didn't even 'need' to implement connection points - for simple builds the collision + grid does a well enough job:
You mentioned that you used these parts some time ago - I guess this was prior to at least one Stud.io update? Have you tried to clear your custom parts folder and try import them again?
Regarding Lego's stand on Duplo parts: I really don't think it's a technical issue, given the relative simplicity of (regular) Duplo parts. Since they didn't bother adding any type of Duplo bricks to LDD back in the day, I believe there is a different reason for Lego not to include them.