N900 keeps lots of stuff preloaded in case they are needed. For instance fast addressbook access is considered essential, as you can do instant searches with just pressing a key in the keyboard. Or you get a call and the phone has to resolve the number to the existing contact name without delay.

Same goes with the camera I suppose; fast access is a top priority.

Preloading is ever more so important as N900 mass storage and swap are extremely slow compared to todays desktops it otherwise so much resembles, and reloading software components would take far too long.