well, for starters, technically you can, but it's on a case by case basis. but realistically, imagine if you made an account on a website and then life got in the way and you forgot about it. then, a while later, you remember it and try to log in only to find that it doesn't belong to you anymore. i'd imagine most people would be upset! even on the case by case basis that taking an inactive username is offered, it's usually (if not exclusively) reserved as a reward for high-level contributors or extremely well-known figures, and even then you're not always going to get approved for it.