No, not really. Brassica oleracea is a single species manually cultivated by humans into variety of forms that are known by different names. Orchids are a huge family of plants. If anything, you should compare them to all Brassicaceae. A quick refresher: Brassicaceae family aside from that notorious Brassica oleracea also contains such things as mustard and horseradish, and even Brassica genus alone contains a whole lot of things that are completely unlike all these Brassica oleracea varieties. And Brassicaceae family is 5 times smaller than Orchidaceae, in terms of number of species.

So this is a silly comparison. Talking about all "orchids" is talking about like 15% of all flowering plants. In fact, I would say that even the mere fact that somebody knows a common English word to name all "orchids" kinda suggests that the family is not that diverse… Except, an even larger family is Asteraceae, and they probably look even more like each other.

(That said, I actually have no idea why all these broccoli, kale, etc are still considered a single species. Can you really easily cross-bred modern specimens of broccoli and kale?)