Why is my Debian jessie always in runlevel 5?

$ dpkg -S /sbin/init
systemd-sysv: /sbin/init

Your init system is Systemd, not SysVinit. /etc/inittab is a configuration file of SysVinit, it is not used by Systemd. I presume you have this file because this is a jessie system which was upgraded from an earlier jessie or from wheezy with SysVinit.

Systemd doesn't exactly have a concept of runlevels, though it approximates them for compatibility with SysVinit. Systemd has “target units” instead. You can choose the boot-time target unit by setting the symbolic link /etc/systemd/system/default.target. See the Systemd FAQ for more information.

If you don't want to use Systemd, install the sysvinit-core package, which provides a traditional SysVinit (formerly in the sysvinit package, which in jessie is now a front for systemd). As of jessie, Debian defaults to Systemd but still supports SysVinit.