The sheer quantity of files that need to be run to create an OS
basically. Using an SSD and hibernating instead of shutting down is
about as close to 'instant on' that a full OS can get to currently,
but even so it would need a full close down and restart for some
updates. As hardware speeds increase the speeds of even a full OS boot
will get short enough to be as close to instant on as makes no
difference :-)

Smartphones / iPads etc are not full OS's, they have a very restricted
operating system compared to an installation of Windows / Linux / Mac
etc.