The short version is that disk I/O is integrated with the page cache with all that entails, and allows things like seeking. To an extent this is a design roote

The short version is that disk I/O is integrated with the page cache with all that entails, and allows things like seeking. To an extent this is a design rooted in a world where disks are much faster than network access. But perhaps that world is coming back with SSD's/NVME/etc..