In a lot of languages, the words for hello and goodbye are the same (for example, shalom in Hebrew, which also means peace).
Each language has its own quirks. :) You just have to go with it.
(Though, if anyone does know more about the history of this for Swedish, I'd love to hear it.)
I haven't lived in Sweden in almost 30 years, so I can't confidently say it is not "doo" in some regional accent, I'm not sure where you've heard it, but I can tell you for sure that at least in the majority of accents, and what is considered standard Swedish, it is pronounced "hey daw" (to my ears, or maybe "hey doh", but definitely not " hey doo").