There's a mixture of reasons, from sexism, audience advertisement, to piggybacking off an already popular series.
Having initially started off liking these, I can say that many of these don't actually start off as married. Some are right before the marriage, or some take a while to get married. However, the tropes remain the same. I suppose, that's why it's popular. Once you read one, you're still craving another of the same type. It's an almost guaranteed success.
Personally, I really intensely dislike the naming scheme. It pretty much points out that the female protagonist is someone else's, specifically a man's (property/wife/girlfriend/consort). The male protagonist is also generally (if not always) stronger and more OP than the female protagonist. Their word is also more reliable, just their presence is a shield for the FL. It's really sexist, but that's typical of Chinese culture.
shrugs