Three reasons:
1. build-package.sh doesn't have the facility to do this:
$ tar -xf /sdcard/Download/postgresql-9.6_9.6.7-0+deb9u1.debian.tar.xz
$ cd debian/
$ grep extension postgresql-9.6.install postgresql-contrib-9.6.install
postgresql-9.6.install:usr/share/postgresql/*/extension/plpgsql*
postgresql-contrib-9.6.install:usr/share/postgresql/*/extension/*
$
while initdb requires extension/plpgsql*.
2. postgresql-contrib takes up only 492kB out of 17.3MB
$ pkg unins postgresql
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
libandroid-shmem
Use 'apt autoremove' to remove it.
The following packages will be REMOVED:
postgresql postgresql-contrib
0 upgraded, 0 newly installed, 2 to remove and 0 not upgraded.
After this operation, 17.3 MB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
$ pkg unins postgresql-contrib
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
postgresql-contrib
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 492 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
$
3. Both Ubuntu and Debian has stopped splitting postgresql-contrib.
It is now only a virtual package provided by postgresql in the 10
branch.
Ronaldo Maia
Fredrik Fornwall
tomty89