I’m confused why they can’t just write a sandbox profile that does the equivalent

Who is "they" ? AFAIK the OpenSSH team focuses on the OpenBSD version and others people/teams use the new releases to create/update a portable version.

So I think it would be up to the team that ports it to Apple, so I think the "Apple Team" is the ones who would worry about sandboxing.

OpenSSH -portable is maintained by the OpenSSH developers, who are also OpenBSD developers.