It's not really that different from any artist having a particular style or motif in their work. They have a certain look that they can become known for, regardless if one likes it or not. Still, in the case of production designers, someone still has to sign off on their work as being what they're going with. If they didn't like it, they'd ask for changes or (in an extreme case) hire someone else to do the job.
And again, I agree with all of that. My point still stands that I haven't seen much originality in starship design in some time, regardless of who the artist is or who is signing off on the work.