(from Memory Alpha)
"The character of Locutus was created as a result of the writing staff of Star Trek: The Next Generation feeling it necessary for the Borg to have a spokesman, which the writing staffers referred to as a "queen bee." (Captains' Logs: The Unauthorized Complete Trek Voyages) Another inspiration on the character's creation, according to writer Michael Piller, was that the writing staff was requested by Paramount to devise a method of potentially writing Picard actor Patrick Stewart out of the series after the third season. This was because Paramount was having negotiating difficulties regarding closing a deal with Stewart that would see him return for the fourth season. (Star Trek: The Magazine Volume 1, Issue 23, p. 16) "
Setting aside the contract negotiation issues...
I think we see here a repeating theme. Folks are correct that, yeah, you could have a character speak to the collective as a whole. But, we are also talking about the mass market here, and maybe cinematographically speaking, that doesn't play well. Your audience is made of humans, and they are used to interactions with concrete individuals. Locutus and the Queen seem to be there because, however it may play as a story to some folks, more broadly the audience may not get it if you don't have a face for the collective.
Heck, the Borg *tell us* this is necessary, in Best of Both Worlds, Part 1:
BORG: Death is irrelevant. Your archaic cultures are authority driven. To facilitate our introduction into your societies, it has been decided that a human voice will speak for us in all communications. You have been chosen to be that voice.
The Borg recognize that the humans need a face to talk to.