I've wondered the same thing. I think that it's expensive to fashion the box for a mould. And I think that they're of marginal use. Before anyone jumps on me about venting the cabin, I will disclose that we have 6 dorade vents and I don't think that they do all that much. When we bought the boat, it was full of mold so the argument that they prevent mold is pretty much busted, IMO. My car doesn't have dorades and it's not moldy so I think that it's a traditional feature that's fallen by the wayside.
With all that said, when we bought the boat, we replaced all the nasty vinyl vents with stainless. It was a large expense, but really jazzed up the boat.
Sabre 38 "Victoria"