Cuz he couldn’t just pour it out. It couldn’t be moved, they couldn’t part it with their hands, couldn’t conjure it away, couldn’t pick it up. Unless it was with a goblet. Why not just pour the potion out on the ground once it’s in the goblet? Maybe it only goes out of the goblet if it’s going to be… drunk. She should have made D try to pour it out of the goblet and have it magically return to the cauldron, or stick immoble to the goblet or something. But then D would have looked reluctant, afraid to drink it. You have to admit the gusto with which he approached the task was formidable. Yeeeahhh… Then why not fill a million goblets? Ummm… maybe… it only goes into the goblet if… there are no bits of itself in other goblets. Yeeeahhh… Maybe you (or your trusted student) could only pick up the horcrux (or fake horcrux) if you drank it. Yeahhh….