The problem is that no-one has incorporated a modification to the Debian installer to have this. There are so-called "preseed" hooks that one can use to manually alter what packages the Debian installer installs. Putting in place pre-seed hooks that permitted something other than systemd to be installed ab initio was one of the first things that the Devuan people did, about two and a half years ago.
But no-one to my knowledge has yet done the work to give the Debian installer this capability without the need for pre-seed hooks, as a menu item/checkbox/question for the administrator to pick/tick/answer. Such work is more than merely writing code, note. It involves testing it too. Thoroughly.
* https://wiki.debian.org/DebianInstaller/Preseed
* https://wiki.debian.org/systemd#Installing_without_systemd
Ironically, this was once a problem the other way around. Attempted changes to enable administrators to select systemd at install time (and indeed upstart, as you can see) failed to make it past a Debian gatekeeper.