Takes time to bring up devices, LOS is a volunteer project, and manufacturers don’t send them devices like they used to. Finally, no matter what they rely on the manufacturers releasing kernel source for a release and some take months and ship squashed and/or incomplete source. Availability of bootloader unlocking is a factor but what I just said is the bigger reason for the delay.

Most modern manufacturers disallow unlocking the bootloader and flashing unsigned firmware, which is a requirement for this kind of thing.

Because it is more profitable for smartphone makers if you need to buy a new one.

Unless there's legislation to force them to allow enrolling new keys or otherwise disabling secure boot, the abuse will continue.