Haha, you know what? I believe there would be only few persons in this world can answer your question. To me, it sounds like "why do chinese people speak chinese?" as well, people hardly know the answer of that. But I assume that in earlier age of forming English pronunciation, past tense of dream "t" ending and "d" ending sounded really similar to each other, and later on when it came up with written phonetic alphabet sysmtem, English just adopted both of them as "dreamed" and "dreamt". Of course, as i said, it's my own assumption. But I really think this is a dumb question..... please don't feel offended...^^!!!