In the context of a master password, it doesn't make sense to change it. It's analogous (closely, but not perfectly) to "master key", a term which non-nerds may be familiar with from the world of physical keys (and which term, in that context, isn't going anywhere, in part because, as with this one, there's no reason whatsoever for it to) so gave normal folks a better handle on what it means and how to use the software.