Because the proper name is
reboot bootloader
.
fastboot
is for userspace fastbootd.
Code:
[nameless@5900x:/tmp/redfin-sq3a.220705.003.a1]$ fastboot reboot fastboot
Rebooting into fastboot OKAY [ 0.018s]
< waiting for any device >
Finished. Total time: 16.216s
[nameless@5900x:/tmp/redfin-sq3a.220705.003.a1]$ fastboot flash --slot all bootloader bootloader-redfin-r3-0.4-8351081.img
Sending 'bootloader' (8762 KB) OKAY [ 0.041s]
Writing 'bootloader' FAILED (remote: 'No such file or directory')
fastboot: error: Command failed
To set the record straight for other people, this is fastboot mode:
If you see this, then you are not in the bootloader and are in fastbootd where you can't flash the bootloader: