there are many theorys some say because they are blind some say they aren't closed, just squinting some say its to hide their eye color some say to not let out giant beams of determination

Personally, I think that it is that since eyes can make people assume gender, and Frisk is supposed to be based off the person playing the game. I think if the eyes were open, people would assume Frisk's gender, which is not what Toby Fox wants. But that is just my opinion.

THOOSE ARE HER/HIS EYE BROWS

The reason is because Toby Fox wanted people to relate to the character,so they have their eyes closed so it won’t make their gender. certain,like everything else about frisk,fun fact:Frisk has no gender,Toby did this so we can decide their gender for ourselves ;3