Because, as my therapist (Dr. Sandra Sammons) put it, "testosterone is a one-way street without a cul-de-sac at the end".
As has been pointed out, even natal women have hair there, it's just usually too fine and soft to detect.
Testosterone affects the coarseness of the hair, and once it's taken effect, there's almost nothing one can do to turn back that effect. That's why TS women have to train their spoken voices for higher registries but TS men typically don't.
There is one woman at my TS/TG group who says she's had some luck with that Nad's gair removal gel -- supposedly prolonged use will thin out and then kill the hair root faster than prolonged waxing (and it's all-natural and non-toxic). She said it took a while to figure out for her face, though, because facial hair tends to grow in varying directions. It's just an option with a cheaper up-front cost than laser.