Probably because the top Google result tells you to do it the way explained in this article. I did it that way for years before finding out about ssh-copy-id.

In a lot of environments, the SSH servers are listening on some non-standard port. I don't see an option in ssh-copy-id to specify the SSH server port.

not sure, maybe not standard enough?

i always use it myself, so much more comfortable

because is not present in the mac default installation

scp is another option if ssh-copy-id is not available