One reason might be that exercises that ask for proofs often have several correct answers. It would probably be just as confusing to a beginning student to come

One reason might be that exercises that ask for proofs often have several correct answers. It would probably be just as confusing to a beginning student to come up with a proof for some exercise and then see the author take a completely different approach in the provided solution. The student still wouldn't know whether his proof is correct or not.