Because many apps will (or would prefer to) only be bundled as Flatpak. I agree that the deduplication is not a trivial problem to solve, even if they might have already solved it for DEBs (I don’t know).
But your entire comment could just as well be a rant why Canonical shouldn’t have introduced Snaps in the first place. It might be good for their bank account, if they can somehow monetize part of the cake, but splitting the cake even further, after it’s already split into DEB, RPM, AppImage, Flatpak, Docker, APK etc., that’s maximum user confusion.