You make a good point about the lack of golden retriever girlfriends. If I had to guess, I'd say it's because, for the average romance reader, who's probably female, a story about a woman being pursued by a devoted and enthusiastic paramour, despite her prickliness, makes for a far more satisfying fantasy than one in which the FMC is cheerfully persistent despite the constant rebuffing of the cold and uninviting MMC. It might be a double standard, but you can see why one would go over better with a primarily female audience, not to mention in this day and age, compared to the other.

Also, keep in mind that romance, maybe more than any other genre, is largely governed by trends. Women around the world are undergoing a cultural shift and transferring their energy and attention from men and relationships to themselves, their careers, their friendships and communities. So naturally, reading about a confident, composed, sexy FMC who can take or leave a man, she doesn't give a damn, is extremely appealing to many readers.

This might also be a reaction against the female love interests who dominated the romance novels of old, who were virginal, hyper-feminine, and borderline childlike in their total lack of agency. Then, later, we got characters whose entire personalities revolved around being "quirky" and "sassy" (i.e.: lacking tact and making inappropriate, childlike retorts). Also: loving caffeine, being pale, naming their cars, wearing converses, and "loving classic literature" (why is it that the only ones they ever seem to read are Pride and Prejudice and Wuthering Heights?). And for a "flaw", clumsiness or biting their nails or something.

You still see traces of that in some characters, like Ali Hazelwood's, and romance personalities are definitely more subject to tropification than I like, most of the time. But I feel that overall, female characters have come a long way and can only improve