Some of it is that it's not an explicit goal. Also, some of that time is spent doing things that are actually useful to some people, or doing things that the system needs to do unless someone tells it not to. For example, it might spend time waiting for hardware to settle, then querying what hardware there is, then loading drivers for the hardware. If you know in advance what hardware there is and how long until it starts up and know that you won't need anything else, you can disable it, but for a generic OS that boots on arbitrary hardware and works, it has value.

Edit: Oh, and let me echo other comments: It depends on your hardware/firmware; the OS can't do anything about it if your machine takes 10 seconds to get to the bootloader.