We do not know that they really do. The scientific jury is still 'out' on whether the purported fossils seen in the martian meteorite are in fact bacteria, or just peculiar crystals. If they are bacteria, they are 100 times smaller than any bacteria we know about on Earth, so I wouldn't exactly say they look so similar. At some point you simply run out of room to have a large enough strand of DNA or RNA to facilitate reproduction. Viruses are about the same size as the martian bacteria, and they cannot reproduce by themselves without a host to infect.