Because there are a lot of different types of filesystems supported. And you'd have to compile them all into the kernel. Which of course you can do, that is s

Because there are a lot of different types of filesystems supported. And you'd have to compile them all into the kernel. Which of course you can do, that is supported by the build system today. But Distros typically prefer to keep their kernels small, and not waste the RAM that would be taken up by compiling it all into the kernel.