It ships nvi, as do the other BSDs. I think it's reasonable, BSDs have a stronger divide between the base system and third party packages, I'm not sure it would make a lot of sense to maintain vim as part of the former.

Besides on my FreeBSD system /usr/bin/vi is around 400kB while on my Debian machine /usr/bin/vim.basic is 2.4MB. It's not much these days but on embedded platform it could still make a significant difference.

Debian is the same way. But as soon as you open a file with vi on a fresh install, you know it isn't vim, so you close the editor, install vim, and then the vi command opens vim.