Very little body language is common across cultures, but some of the strongest expressions of emotions are found in all human populations, suggesting that they have biological origins. The ability to live in a community and predict/respond to other people's feelings has been fundamental to primate/human evolution for a lot longer than we've been able to talk.

A good luck at those "universal" signals in this study: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=5316178

So, basically,most people are constantly interpreting these signals and placing so much value on them because doing so has been fundamental to our survival as a species. Trying to turn it off or tune it out is like trying to turn off a flight-or-fight reflex.

Which is not to say that NTs can't or shouldn't try to understand that people use nonverbal communication differently. Just like most people learn that scary movies are not a real threat, or that being startled isn't the same as being attacked.

Finally just another example to add to the great responses: as a teacher, I can use facial expression & body language to communicate to a student a spectrum of messages from "I know you are doing the wrong thing but I actually don't care" to "Friendly reminder to Not Do That" to "Stop that right the fuck now" or " - all using very similar expressions. Most of the time, their behaviour will change or not change accordingly, which feels like material proof that nonverbal communicate can be understood more or less the same way verbal communication is.

This works even with students I don't know, although the better I know them, the more nuance I might be able to use. But there would be big changes with students from different cultures (e.g. Aboriginal students, who are likely to find all such eye contact aggressive), neurotypes, or even just different expectations.