Back in the '60s, that was part of the hype. Few of them ever went on to do anything else, but it didn't matter. Audiences wanted to see the new girl, the new gadgets, the new exotic location, etc.
I'm sure the savings on using an unknown are a plus to the producers, as well.
I actually though Teri Hatcher was okay, though. Not in the film for very long, but she seemed to have a more mature connection to Bond that wasn't used very effectively in that film.
Denise Richards and Halle Berry, though... pretty bad. I can't believe the franchise was so off the rails at that point that they even considered spinning Berry's character off into her own film.