More seriously, metallic bars on vibraphones and similar instruments are made of carefully elaborated alloys, and meant to sound nice when played with soft/hard mallets within a certain range. They would not maintain their integrity if hammered mercilessly, and would most probably sound like utter crap before they give up the ghost for good.
On the contrary, some metallic parts like short tubes or similar pieces (cf. Xenakis' Psappha for example), while being relatively small in size, are somewhat heavy and literally unbreakable due to their thickness and hardness. They will take a dent if you have anger issues but that's about it. Think construction site material… The sound is distinct too. Sometimes they're so thick that dampening is not even necessary.
