Signal should change this, but it's typical of the traditional desktop OS security model in which applications running under the user's account are considered trustworthy. Security-oriented software like Signal should take a more hardened approach, but this is not some glaring security hole.
Maybe its time to rethink desktop security. I realize that there is credential manager on windows, keychain on mac, and similar on gnu/linux; even with that it seems for a lot of services "all" you need to do is steal a cookie and all of a sudden you are someone else.