Because it's confusing. And it'd s confusion I swe expressed by other fans. The Borg have one terminology for individuals and groups.But... it's their name.
Although even still, i'll amend by examples.
Rather than "Romulan Empire", the Romulans... as an organized entity, are no longer a threat. Romulans, as individuals or small groups, can be.
Borg, as an organized entity, are no longer a threat. Borg, as individuals or small groups, can be.
The Borg Collective is no longer a threat. Borg can be.
It's a really weird semantic game we're playing here...
"Forget about the Borg. We really need to be worried about the Borg."