Because they already do, to some extent. Now this is my own experience and intuition, but I would say that at least 15% of people's photos on dating apps are doctored in some way. I know some might dispute this, but I've seen enough questionable artifacts in such photos that I believe quite a few of them are artificially flattering. This isn't even including those puppy-face photos. And yes, already there are people who outright steal other people's photos. I've been on many dates where the person looked nothing like the photos.
There are a few problems with these kinds of fakery. If you are simply touching up photos to make them more flattering, that takes a lot of work that the average person doesn't want to do. There's AI that can make you look sexier, but said AI is often inconsistent, and isn't that good at arbitrarily modifying features. People who might want to use photos of people who aren't them may have a moral hangup from using someone else's likeness, and that prevents them from going ahead with it.
Deepfake technology solves both of those problems. You can use it to change your eyes, your hair, the shape of your face, consistently at every angle, and possibly I real time. More people would feel comfortable using a likeness that isn't theirs because they don't feel like they are stealing, especially if they have no intention of meeting people in real life.
I'm not saying that all or even most people would use this technology in dating, but it would require only an appreciable portion of an audience to be dishonest to cause many people to throw in the towel.
These are just my crazy theories, so maybe I'm way off base. Soon we will have deepfaking that is easy to use and requires minimal training, and I believe they will be used often and eventually be normalized in society.