Because it’s not their responsibility to add a feature people primarily use on servers to an installer built for desktop usage. Because there installer isn’t bad, it’s loved exactly for the ability to automate it. Because their installer works, and it doesn’t take a lot of manpower. According to debian salsa, it basically only receives translations and package updates, some of that automated.
Why have the debian devs go off and add support a whole another installer (by support I mean actually attempt to add features to it) when they have a perfectly nice, working installer? The devs have more important things to do.