That was the whole point.
The Borg specifically said that they assimilated Picard so that he could speak for them and facilitate Humanity's assimilation. In ST: First Contact they expanded upon this by revealing that the Queen wanted "a counterpart," but even in Locutus's first appearance his role is clearly more than that of a simple Borg Drone. He is supposed to be, in a twisted Borg sort of way, almost an ambassador, or perhaps a provincial administrator for a conquered territory. Picard later refers to Locutus as a "liason."
That means the Borg don't just want to take his raw knowledge, they also wanted him to serve as an intermediary or spokesman of sorts. Given that objective, it makes sense to refer to Riker in a way that Picard would naturally. Dropping the familiarity and personal knowledge that Picard held would have defeated the entire purpose of Picard's assimilation. They could hack a terminal if they just wanted everyone's names, and they'd already ordered the Enterprise to stand down and be assimilated through an impersonal audio communication. It didn't work, so they were changing tactics.
The Borg didn't want "Drone 89240931 of 95152645" to talk to "Commander William Thomas Riker of the United Federation of Planets."
They wanted Locutus to talk to his Number One.