Romance is wish fulfillment in the purest sense. The reason why most authors cant write past certain point is that they were never in a relationship or all their relationships fell apart, and they have no idea how to portray a successful one.
To me, that always sounded more like an excuse by some authors who disliked research.
Going by that logic, most authors also shouldn't be able to write a fight scene or a science fiction novel or magic or whatever. We also have no experience throwing a fireball.
I don't know how many authors have been or haven't been in a relationships. But at least younger authors will mostly have less experience, that's true. But one can easily learn about writing good relationships just through research or reading other romance stories. The problem is more, that due to their own experience readers will see flaws in relationship writing faster than flaws in fight scene descriptions and the work inexperienced authors have to invest is massive.
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That said, romance as a genre is also based on a simple "Will they/Won't they" question. And based on the type of romance, that question will be answered when they get together or when a divorce happens or whatever. The "romance story" is over at this point.
So if you want to continue your romance story, you'll have to introduce a new question. And the question for the relationship would be "will they break up or not". So your romance novel would do a 180° turn from "they finally kissed and are together" to "will they break up or not"? There are a lot of YA-series who suffer this problem:
- Volume 1: Happy and together
- Volume 2: Weird problem out of nowhere and a forced "do they still love each other" plot
- Volume 3: Even worse problem and we repeat the "does he still love her" question again
....
The reason the romance often takes a backseat after the confession and turns into a romantic subplot is simply this. You don't want to throw in weird drama just to continue writing a romance. Everything after the initial question is a lot of slice of life where outside stimuli and time (knowing each other better) will be the "antagonistic force" towards the couple.
This is a lot easier to do when you have a different plot that keeps the readers' attention while the romance is slowly developing. You can have a different hook for an arc/a volume and only throw in one or two scenes of development. This will make characters look a lot more fleshed out compared to "Yesterday I loved you but today I hate you!".
There is an audience for pure slice of life we live together and every few days we learn something about each other and sometimes we even have a quarrel. But this isn't something build for mass appeal as it is slow paced and character driven (mostly internal conflicts). There is an audience, but to a lot of authors and publishers, the amount of work you have to put in isn't worth the outcome when you can have better numbers with a romantic subplot and some weird "angel-devil-drama" on top of the YA-clichés...