I think they probably need a special-cased night time neural net or something, and they haven't gotten around to it. Obviously, we can tell as humans whether or not it's raining at night. But a wet windshield at night looks quite different than one during the day.
So far mine are working well at night. I do live in an area with well lit freeways for the most part. Have you tried RainX? RainX makes the water bead which does two things, you need wipers less because the water blows off and the camera has a better drop shape to see.
Mine seems to work just fine at night, though there is always some light from other cars, street lamps or even my own headlights. How dark was it when your wipers don’t work? If it is that dark where nothing is visible, then does it matter if the wipers work or not?
Same goes for Elon, hence designing a rain sensing function is a completely foreign and abstract concept to him. He also doesn't believe in using someone else's proven solution (too lame, why not reinvent the wheel every time). Unless Mars climate matches California, I foresee a lot of issues with Elon's adventures on Mars.