Why are bootloaders in rewritable storage? I successfully flashed my device a few times, but I am still paranoid about hard bricks. I am sure I can use my Arduino to fix them, but this made me wonder why bootloaders are stored in rewritable storage. I am aware that there is a PBL and an SBL and all that, and the PBL is in ROM. In this context, "rewritable storage" means a storage medium that can be electronically rewritten (EEPROM, Flash, RAM, etc) and "ROM" means read only storage, not aftermarket Android distributions such as Lineage OS.

Summary:
Why would code that is required to even turn the phone on, not to mention booting Android, be stored in a location that can be rewritten.
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).