It's not in the kernel. It just comes a long with the kernel. You can compile any of the drivers as modules. Back in the day when you had to fit your kernel and boot loader on a 1.44MB floppy. We would save space by compiling most of the drivers as modules and then they would get loaded into kernel space on boot. Now a days, a 100MB kernel is not a big deal when systems have Gigs of ram and harddrives are in the Terabytes. They keep the driver code with the kernel code mostly for the reasons that @dafta gave. When I was a Windows kernel dev for Intel, Microsoft did the same thing. That's how you get inbox drivers. As a Windows kernel dev for Intel, it was our goal to get our drivers inbox'd with Microsoft so their developers would be responsible for maintaining the driver code, as well as testing, when ever there were changes to the Kernel that affected drivers.