I have a question. If OS X is Unix-like, why does it have to be restarted almost every time I do a software update?

I have a Linux server (also Unix-like) that has been up for hundreds of days at a time without having to be restarted for fairly major updates including updating PHP, Apache, mySQL, the hostname, perl and dozens of other software packages.

What is it about OS X that requires it to be restarted for updates?