I think/assume OpenBSD is mainly used as a server OS. Yes, passionate people use it as a desktop but those mostly read the FAQ anyway.

Currently and as far as I know, bioctl does only support user typed in passwords or key disks. You certainly want also encrypted disks on your server but requiring user typed in password is oftentimes a no-go (think of various firewall appliances doing a reboot and not having remote hands). A compensation can be the key disk but I don’t know how widely that is used.

Hardware bound encryption like with a TPM is not supported. Also Linux is still exploring here as far as I can tell (no installer offers that).

In sum: I think disk encryption in the current form is not a tradeoff many installations will take.