TLDR: Because there's a market for them.
Having sadomasochistic kinks isn't uncommon and has, in fact, been around for a good long while, so I assume that what you really want to know isn't so much as to why it's romanticized but more on why people find it stimulating. Well, psychologists have proposed a number of reasons, most popularly, that they need a way to vent out their stress or feel intimacy by allowing themselves to be vulnerable.
A more straightforward hypothesis--and one I prefer, tbh--is that there are people who has associated dominance and submission with eroticism in some form or fashion during the course of their lives. Simply put, they relate vulnerability to sex and not necessarily because they're stressed or could not feel intimacy otherwise. If you wanna read more about that, I recommend reading about fetishism in psychology (since you'll also find the term in anthropology and literature, both of which won't answer the question).
As for the point made by others about the vilification and romantization of the trope, that largely depends on the current norms and trends of the times. If you're interested, Foucault, Freud and Lacan are the most popular ones and are a good start as primers for how this trend has been viewed and studied by the public over the years, and then leading up to more modern studies and interpretations like Falaky and Newmahr. Although you'll find them sounding more hypothetical than anything, they are at least reliable enough that at one point (though I suppose that would be now), you'll be lead to the Diagnostic and Statistical Manual of Mental Disorders where you'll find sexual masochism and how it's seen by the medical community and its forensic classifications which has been revised over the course of the years as more studies and data were gathered about it ranging from relatively subjective questionnaires to the more objective spinal nerve stimulation, MRIs and EEGs.
Generally, since punishment largely falls on men, you'd find most male-oriented literatures vilify the act unless otherwise justified in some sense, e.g. revenge plots. On the other hand, female-oriented literatures usually establish the image of prim and proper ladies prior to being "ravaged" by the ML. It's the prim and proper they often put emphasis on and it would seem that the trope is there to subvert that imagery.
The hypothesis I find the most disdainful uses Darwin as pretext. Supposedly, dominance entails reliability and therefore, better genes. In other words, women are supposedly biologically inclined towards more aggressive men. Well, I'd rather people read up on this than have to explain it myself since, as I said before, I'm disdainful of this hypothesis so I'm likely to be the worst person to be explaining it. I like to think, however, that my disdain is not unwarranted. They liken it to how physical attraction plays a role in romance, saying that certain characteristics are associated to better genes and therefore, better partners. While I agree that this is an influence, this has nothing to do with social roles. Unlike the other hypothesis, there is no way to objectively verify this statement at all and rely on other observable phenomena as basis without measurable correlation between them other than, "This seems to be the way it is, so this is probably how it is". Well, I'm rambling on my own now so again, research this on your own as I'm not the best person to elaborate on this.
Finally, to the last point about this being shrugged off as fan service, that is because it is how they usually appear to be. If the manga advocates and actively encourages the act, however, then it's a different story. People usually react strongly against characters that are scumbags through and through, but the Leads (capitalized for the sake of differentiating) for these types of fantasies often have redeeming traits about them (e.g. actually being caring, having a gentle side themselves, or reflecting and regretting their actions to some degree, etc). They often make a clear distinction on why they're okay with one character doing it and not the other.
Point is, fan service is when the manga goes through lengths to justify it or makes clear boundaries of what is or is not okay. Otherwise, you'd have to wonder about the author and to some degree, their readers' moral standards. Unless it's hentai. Then you should expect all logic should fly out the window.
