Why Do Software Updates Require Reboots?

When I started working with Linux about 5 years ago some technical experts I spoke with suggested that Linux/Unix OS's were superior to Windows in many ways, one reason being that software updates could be performed without rebooting. The end result is a system with uptime that can't be matched.

When I purchased my Mac I thought -- the next time I reboot this system will be after installing Leopard! Since then nearly every software update has required a reboot of the system to complete. Do the updates that Apple pushes out to our systems really require a reboot or are the patch engineers at Apple just playing it safe? Is it possible that the software update application doesn't have the ability to restart the necessary modules to fully update the system?