I enjoy people bringing up some meta discussions referring to the Discworld once and again, and never thought of the question as such before.

It would be interesting to be pointed towards an example of what you thought was well written romance in a book you liked, superbanana, because your first statement kind of rang a bell with me, but on closer inspection I went down the "Yes, but..." -road.
I don't have much time for in depth analysis here, but I think to some degree, what some readers may see as a lack of romance -you can read it that way - is owed to the often ironic tone used to describe human nature throughout the books.
The whole writing style and point of view just don't make me hear the "Flower Duet" in my head - a beautiful piece, but out of place in that scenario. In many of the novels, especially the Watch novels with a protagonist as dryly sarcastic as Vimes, it would come across as out of character and as a breach of style. It's similar with Carrot and Angua - Angua starts out wary of all of her relationships, after having been chased out of some of them and several cities, and Carrot is incredibly formal and dwarfish in a way - he thinks now that he's told Angua he loves her, he doesn't have to tell her again, after all, it's been settled. Not a romantic.

And even in real life, not only in (well written) novels, people may experience change as characters, but they don't drop out of their character all of a sudden. They stay themselves even when they fall in love. Their voice and narration stays the same, and many of Pratchett's characters would understate grand feelings - love is not a universal force that makes everyone sound and act alike. If you are together with someone who doesn't talk about his/ her feelings, you may develop a relationship that works with small, everyday proof. Vimes is shown clinging to Sybil's cigar case, to the little things they do for each other and the rituals they develop. Big feelings would have been out of character for both.

What is universally true for all romantic relationships is that in the books, they are all side stories, and falling in love, if it happens at all, doesn't become a thriving force for the characters or the plot all of a sudden. That is, maybe, why even the heartache or butterflies in the stomach of the more emotional characters isn't being given much room. If it would, the novel would indeed have to become a love story, and Terry Pratchett may just be an author who doesn't write books that are mainly love stories.