I think locking the bootloader can prevent rogue shops installing malware (I heard this was an issue in china). But if that is the reason, manufacturer could give devs and consumers method to unlock.

I did wonder then if locking bootloader could be at request of govt (cue Huawei paranoia).

Perhaps it is a feature not used often enough to justify costs of warranty service/complaints to manufacturer (kids bricking their phone).

What Xiaomi does is you have to login with a Mi account and use it for a week, and then a bootloader unlock can be issued.