It is not installed by default. The way it works, Debian provides you with many thousand of packages which you can install with one command. Debian makes sure that all programs and libraries work together, and that nothing is downloaded more often than necessary (by factoring out dependencies).

This approach works very well, but it has a few shortcomings, including the fact that sometimes, Debian ships a somewhat outdated version of a package, which may contain bugs. Some bug fixes, such as security updates, are pushed to users, but "non-critical" bug fixes need to wait.

In particular, that approach is quite nice for servers. If you're on a particular Debian stable release, you know that anything you install will work with the same base set of packages, and any security updates will work as drop-in replacements. You're never going to find that installing a new package suddenly triggers a set of cascading upgrades as it pulls in newer versions of libraries and other packages. If something depends on MySQL, it'll be the version of MySQL in the Debian you're using, and you won't be forced to do an unplanned MySQL upgrade in order to install the package. Then you can do planned, infrequent upgrades between the major releases, when you upgrade from one Debian version to another.

(Not that Debian's unique in that. FreeBSD's release branches and Red Hat Enterprise Linux use a similar model.)