SFTP: changing the owner of a file / directory without changing the group

In SFTPv4+ there's an SSH_FILEXFER_ATTR_OWNERGROUP attribute flag that
takes in strings. In SFTPv3 and earlier there's a
SSH_FILEXFER_ATTR_UIDGID attribute flag that pretty much does the same
thing but takes in integers.

My question is...  why are the owner and group being set at the same
time? On Linux it's not chowngrp - it's chown and chgrp as separate
commands. Why aren't they separate values in SFTP?

I guess if all one wanted to do was set the owner one could do
SSH_FXP_STAT to get the the group and then update the owner to the new
owner and update the group to be the same group that it was before but
that seems like it ought not be necessary - that if SFTP were still
being developed as a protocol that that's an area that could benefit
from improvement?