I installed iode onto my FP3, and I could relock the bootloader.

This depends on the devices. Some of them like pixel and fairphone ones allow bootloader relocking, lineage supports this, and iodé too.

I think that when people make the transition to a custom rom, they often don't care about that because they primarily want to give a second life to a slightly outdated phone. Then, some of them become more conscious of the possible danger of having an unlock bootloader, and choose next a smartphone with relockable bootloader.

Why?