Why are bootloaders in rewritable storage?
Bootloaders, like other partitions, might be updated officially for various reasons:


Switching to a new partition layout (e.g. a bunch of older devices with life spanning across Gingerbread and Ice Cream Sandwich, the latter requiring considerably more space)
Implementing new security mechanisms (e.g. earlier Xiaomi devices, old bootloader readily unlockable, new bootloader requires logging in and verifying before unlocking)
Ensuring you're on a version the manufacturer wants you to be, i.e. preventing downgrades (e.g. Samsung, doing so will halt halfway).