Because you can tell a good story in a few words (althoguh frankly, I've skipped quite a few of long-winded dialogues in PS:T). But if you want an engaging romance, you have to build up the interactions between characters somehow, and honestly, I would not play a game, which is overloaded with dialogues.
I'm going to just ignore that part about PS:T. Jesus Christ...
In reality romances (and definitely sex) can happen rather spontaneously without huge amounts of dialogue or build up. Two people meet somewhere, something clicks, they might have sex (or not), and then it possibly develops into something deeper later on, sometimes rather quickly. It's not like you need to have extensive dialogues about philosophical ideas, daddy issues, your past lives and other shit before you're ready for bed or even a longer relationship. I don't know where people even get the idea that you'd need to have an incredible amount of build-up for something like that, because a little spark is often enough if there's mutual attraction of some kind. Even books and movies generally manage to portray romances with relatively few words, sometimes even successfully. Apparently BioWare have moved on from the "sex is the final stage of the romance" structure of their romances, but the dialogue is still written by fucking aliens who've never hit on a woman or a man at a bar in their lives. A good computer game romance is a question of writing, mostly, and usually computer games just don't have very good writers.
Many people shit on The Witcher's "immature" approach where Geralt fucks everything that moves and has a vagina, often in inappropriate places and situations with maybe a tiny bit of foreplay and very little talking, but I find that much more believeable than what BioWare's doing, and definitely preferable to it. It's often funny and there's at least some spontaneity and light-heartedness to it whereas BioWare either treat sex as a super serious thing like good Christians or write "playful" dialogue that wouldn't feel out of place in some terrible porn movie. Some people mentioned Max Payne 2, and that's another game where there's not a whole lot of dialogue between Max and Mona in the entire game, but their relationship feels natural and believeable nonetheless.
To be fair, I wonder if there even exists a writer who could write a romance that would satisfy BioWare's target audience without being godawful in some way.
