Because how would it know that some little detail about some configuration has changed (maybe you moved a PCIe card from one slot to the other, a firmware of so

Because how would it know that some little detail about some configuration has changed (maybe you moved a PCIe card from one slot to the other, a firmware of something was updated, or whatever)? It would have to scan everything, every startup...and that takes time... And because the BIOS isn't your only state-holder, anything with a controller on it inside your PC holds state, and that is basically everything today. Including external devices, like everything connected via USB (actually those have two controllers minimum, a USB controller which holds communication link state and a controller for the actual device function, like being a keyboard or mouse). It totals to hundreds of controllers, and it simply isn't feasible to standardize a way for this entire system to somehow store an overall consistent runtime state. That is why plan B is taken: just re-establish that state every time. We call this process "booting" or "initializing".