Development teams do both at the same time, writing new code and fixing the old one. It's not one or the other. Taleworlds is not a single autistic person who can do one thing at a time.
If your favorite bug isn't fixed for a long time and you think this is inexplicable, it could be that they were planning to rewrite that piece of code from scratch (programmers love this), but they didn't get around to it YET. This in practice is a silly excuse, but people keep making it so they won't need to fix things. Or the person who is supposed to do this has other critical tasks, and others are incompetent or good at avoiding chores like bugfixing. Or each one of the devs is simply evil and doesn't care about your feelings.
Hope this helps.