They’re aware of that desire. They’re also aware of your desire for security fixes, feature updates, drivers for new hardware, etc. And they’re also aware of your desire to upgrade your computer to the next OS version for $99 instead of throwing it out and replacing it. (Not to mention their desire to sell you that $99 upgrade.)
Of course you can burn an OS (or just certain routines) into ROM and just replace bits as needed off disk. Classic Macs, and many other systems of the time, did this. But in the end, it isn’t worth it. I remember I once traced my Mac IIsi to see how many of the system calls that were made went to ROM routines and how many to disk-loaded, and it was close to half and half. Once I upgraded to System 7, it was more like 1%/99%. So, people have mostly abandoned this strategy.
Besides, loading stuff up from disk isn’t really the delay. Waking a computer from hibernation requires a lot more disk loading than booting, but it takes 1/10th the time. A big chunk of time is spent surveying your hardware to figure out what needs to be loaded (the fact that this is typically done twice–once by BIOS or EFI, once by the OS itself–makes it even worse), and another big chunk is spent actually running the drivers to get them set up.
And to some extent, this is necessary. On a computer, unlike a camera, you can add/remove/replace almost anything between shutting down and powering up, and you still expect the computer to boot up next time without having to go through a complicated manual configuration procedure.
There are all kinds of tricks to get around this. For example, they can assume you’ve got the same hardware you did when you shut down, unless there’s some unexpected error, and wait until the OS is up and running to start looking for new or changed hardware. Some bits can’t be handled this way (usually because the firmware insists on running every time, or because an unexpected error would be catastrophic and unrecoverable), but they can (and do) still shave seconds off boot time this way.
Things are getting better–computers boot faster than they used to, and (as beowulff points out) they usually don’t have to do so as often. And EFI gives the OS manufacturers more tricks to improve things, so things will continue getting better. But computers will never cold-boot as fast as cameras.
While I can’t remember the last time I rebooted my MacBook Pro, my first-gen MacBook sometimes has a video glitch after recovering from sleep that requires a reboot to fix. (And I just had to reboot my iPhone earlier today, because some upgrade that was only supposed to require a respring ended up getting into an endless respring cycle.)
Likewise, there’s a reason I remember the SysRq-SUB sequence by heart for linux; it isn’t needed that often, but it does come up occasionally.
And most people run Windows. My last ThinkPad had about a 50/50 chance of needing to be power-cycled after it was undocked and redocked, and I can’t remember the last Windows computer I had that went a year between reboots.
