As a user, I don't want to assume that every single maintainer of every single app that uses (a statically linked) libfoo is keeping up to date with security issues in their dependencies and has the time and ability to promptly update their software.
But I feel pretty safe believing that the debian libfoo package maintainer is on top of things and will quickly release an update to libfoo.so that all apps running on my system will be able to take advantage of.
Specifically in the case of Debian, who is going to pay for all the additional infrastructure (build servers) that switching to dependency vendoring/static linking would require?