It’s because the Old Testament is actually just the Torah, rearranged and edited to fit the beliefs of what was once a sect of Judaism. That sect branched off when they decided that Jesus Christ was their Messiah, then progressively became more open and split away from the rest of Judaism and became their own religion.

That might be a bit oversimplified, but that’s really the gist of it. Jesus made a new covenant with god, which was meant to replace the old one, chronicled in the New Testament; but the old covenant was kept in as background, becoming the Old Testament.

Some of them do… Or at least, they did - I mostly remember seeing it when I was in high school, and it sort of faded out when I was in college. Not sure if it fell out of style, or if it’s only a thing with the Hot Topic/mall goth crowd

Edit: uncorrected an accidental autocorrect