Hardware requires specific handshaking to get it to initialize, the kernel cannot assume that the hardware that was present last time is available this time, and orchestrating everything is not an easy process.
The holy grail would be a kernel organized and designed to cache everything to disk related to its own configuration, and only re-execute/rerun the hardware reinitialization code.
Admittedly, Windows' current approach to bootup is very close: it closes all applications then pseudo-hibernates the kernel. Bootup simply reads the hibernated kernel state from disk and reloads userspace.
This is of course besides traditional full ACPI hibernation, which is pretty cool but isn't a perfect art. (I'm typing this on a ThinkPad T23, a fairly old (and thus well-tested) hardware configuration; its hibernation/resume is usually rock-solid, but this morning my WLAN decided to get all stupid with dhcpcd, and USB decided that while it could see my external HDD, there wasn't a disk inside it. From-scratch boot is still the only way to get a reasonably predictable system state.)
The holy grail would be a kernel organized and designed to cache everything to disk related to its own configuration, and only re-execute/rerun the hardware reinitialization code.
Admittedly, Windows' current approach to bootup is very close: it closes all applications then pseudo-hibernates the kernel. Bootup simply reads the hibernated kernel state from disk and reloads userspace.
This is of course besides traditional full ACPI hibernation, which is pretty cool but isn't a perfect art. (I'm typing this on a ThinkPad T23, a fairly old (and thus well-tested) hardware configuration; its hibernation/resume is usually rock-solid, but this morning my WLAN decided to get all stupid with dhcpcd, and USB decided that while it could see my external HDD, there wasn't a disk inside it. From-scratch boot is still the only way to get a reasonably predictable system state.)