"Go get a proper application for that" entails a lot of work, including things that can be cognitively difficult like sifting through the giant sloshing sea of half-finished code that is the Internet to pull out a few good candidates. So it's not hard to understand why using a tool you already have (and already understand) would be more appealing.
I suppose it doesn't have to be a proper application; I've seen people manage to-do items in Windows Notepad and it seemed to work for them, God knows how. I tend to think in terms of task list management tools because I've gone to the trouble of setting up a CalDAV server and integrating all my computers and devices with it, but the general point could (and, as you point out, really should) be less "you need to go find an app" and more "try thinking about to-do lists in any terms other than email, because anything else will probably be less painful".
"Go get a proper application for that" entails a lot of work, including things that can be cognitively difficult like sifting through the giant sloshing sea of half-finished code that is the Internet to pull out a few good candidates. So it's not hard to understand why using a tool you already have (and already understand) would be more appealing.