It’s not because they are so much more expressive than non-signers!
American Sign Language (and other sign languages….yes other ones…there are about 300 different sign languages throughout the world!) use the face for grammatical purposes.
When I say grammatical purposes, I mean that they are required for the sentence to be considered correct. In English we signal that something is a yes/no question by rearranging the words. For example, if the original sentence is “That is my sandwich.” we flip the “that” and “is” to make “Is that my sandwich?”. Sometimes we even need to add extra words. In the sentence “You like sandwiches” we add the word “do” so it becomes “Do you like sandwiches”.
Instead of mixing up the word order, ASL signals questions with the eyebrows. Yes/no questions have them raised up and WH questions have them furrowed down.
We can also modify the mouth to show word gradients. The difference between big and humongous in English is just a different word. The difference in ASL is the way you move your mouth while signing.
Another common use of the face is in topicalization constructions. English doesn’t use this sentence format often but ASL uses it a lot. In English this would be a sentence like “That sandwich you made, it’s my favorite”. That first half is the topic and it’s brought to the beginning of the sentence. In ASL it’s also brought to the front of the sentence but it’s also accompanied by raised eyebrows.
Spoken languages are limited by the number of articulators. In other words, there’s only one mouth so you can’t do a bunch of things at the same time. We’re pretty much limited to sounds coming one after the other and some overlapping intonation.
Signed languages on the other hand (pun intended) have 2 hands, eyebrows, eye gaze, the mouth, and space. We can have each of these pieces doing something different all at the same time!
This simultaneity of American Sign Language can seem overwhelmingly chaotic to people not used to it but if you take a closer look there’s a method to the madness!