I’ve used Arch Linux and openSUSE Tumbleweed in the past and I have been using Linux for over 10 years …
With each new version of an application there’s the change that configuration files or functionality changes. Packages might even get replaced with others.
You would be surprised how much changes between Ubuntu LTS versions … My archived Ubuntu installation script had lots of if-statements for different versions of Ubuntu, since stuff got moved around. Such things can be as simple as gsettings schemas (keys might get renamed), but even these minor changes make documentation and therefore reproducable reinstallations troublesome.
With a fixed release all these changes are nicely bundled in one large upgrade every couple of months/years, which makes it easy to document and to plan when to do the upgrade.