Why do you think certain contractions never get used by themselves? For example, you might say "I'm hungry". But if somebody asks "Hey, are you hungry?" You'd never respond by just saying "I'm" (you'd say 'I am' or 'yes' or 'no' or maybe 'I'm not'). Or you might say "I'd like to go to the movies." But if somebody asks if you want to go to the movies, you wouldn't just say "I'd".
in those examples, the second word needs to be emphasized, because the entire question focuses on whether that word is positive or negative. and it is impossible to give a word its own emphasis when it is part of a contraction