Imagine a fully statically linked version of Debian. What happens when there’s a security update in a commonly used library? Am I supposed to redownload a rebuild of basically the entire distro every time this happens, or else what?
Then you update those dependencies. Not very difficult with a package manager. And most dependencies aren't used by a ton of programs in a single system anyway. It is not a big deal in practice.
Libraries already break their ABI so often that continuously rebuilding/relinking everything is inevitable.
Steel-manning the idea, perhaps they would ship object files (.o/.a) and the apt-get equivalent would link the system? I believe this arrangement was common in the days before dynamic linking. You don't have to redownload everything, but you do have to relink everything.