Because in quite a few distributions you don't get security updates reliably. For example Debian Stable excludes most WebKit-based libraries from their update policy.
https://www.debian.org/releases/stable/amd64/release-notes/c...
So users of browsers like Midori and Epiphany or E-Mail-Clients like Evolution on Debian Stable, currently end up using a WebKit library that hasn't been updated in more than a year.
The same issue applies to Ubuntu and most of its derivatives as well. E.g. Ubuntu 14.04 users get a WebKitGTK+ library which hasn't been updated in almost a year.
http://changelogs.ubuntu.com/changelogs/pool/main/w/webkitgt...
Of course, the user doesn't get a warning dialog when he installs applications which rely on those outdated and insecure libraries.
Ubuntu users also shouldn't rely on packages from the Universe repository (which are by far the most packages), if they care about security. Those packages are community maintained and often don't get a single update in years. In the past they didn't even update Chromium reliably.
BTW, it is possible to check in Ubuntu which packages are supported or not: http://manpages.ubuntu.com/manpages/xenial/man1/check-suppor...
How many times will this have to be repeated? Stable is for servers. If you're running webkit based libraries on your server you have other issues. For desktops, both Testing and Unstable are the valid options.
If you check the Debian web site you'll see stable is the only one which is offically supported and recommended by the project and there is no distinction for "server" or "desktop" use cases. Had stable was non-suitable as a desktop OS you can be sure Debian developers wouldn't bother releasing and supporting thousands of desktop/graphical packages with the stable release.
In the end stable, testing and unstable have all their pros and cons, strong and weak areas and some are more suitable for some use cases. That Debian stable is only for servers and it is not a good desktop OS is a myth that needs to die. Stable is a damn fine desktop OS. Everyone is free to use whatever they deemed best for their use but when you post blanket statements like "You. Are. Not. Supposed. To. Install. Debian. Stable. On. A. Desktop. Machine." and "Stable is for servers" on a public forum you are spreading misinformation and you should just stop.
Why is getting the latest security updates giving a false sense of security exactly?
> The File Manager UI, for example, lacks too many features and user must investigate alternatives and either assume that everything is all right or deeply examine security vulnerabilities for each available option. Same goes for basic things like text editor or calculator and so on.
This is where you're being unfair, the notion that the default file manager is not good enough is subjective, it is plenty good for most people.
(Finder for macOS also lacks many features, yet many people never bother with alternatives).
Moreover, if you do need to find a replacement, if it is in the official repos, it probably means it is popular enough to be solid.
As for things like an editor,, are you telling me that Notepad is more featured than gedit?