I never really thought it was Brosnan on the covers. I always think actors like to have their face shown & not be in a silhouette unless it's for some dramatic reveal in a movie.
They were done in 1982, though, when Brosnan was a jobbing actor, and far from being a star. They could have just been a reasonably well paying gig. He'd apparently met Cubby Broccoli in 1980 during the filming of For Your Eyes Only, and had come away with the impression he had a chance at succeeding Roger Moore as Bond, but he didn't have any profile at all. If he had set his heart on getting the role, as it seems from subsequent events he had, it makes sense that from 1980 on he would seek out and accept jobs that made him look the part and thus be more likely to get it. Remington Steele was obviously the big move in that direction, but being the guy on the cover of the new James Bond novels would also seem like a good idea to cement Cubby's feelings he could do it.
It's also possible that Brosnan
did want his face shown on the covers, but that Berkley didn't want to associate the character with one model/actor that much. Or even that he wanted his face shown at first, but then had second thoughts that it might scupper the Bond deal for him. Perhaps less likely. But that he did the job, and his agent then leaked that he had done it to raise his profile for Bond? Well, stranger things have happened, and George Lazenby, Findlay Light and Rikki Lee Travolta could probably tell you about them.

All speculation. It would be nice to ask him and find out! I think there's a story there.