why do we use the word 'guys' as a gender neutral term?

the other day, I was talking to my parents and referred to them as 'guys' (as I joke bcs people don't rlly call their parents guys so it was ironically but besides the point) and my mum pointed out that she isn't a guy so why is she being referred as one when in a group? and it really made me think like why do we get to call a group of people who are either not exclusively male or even ALL not male 'guys' but we couldn't call a group of ACTUAL guys 'girls' ? or 'ladies' ? it doesnt make sense to me. 

now u might be thinking 'katya, there's so many worse instances of sexism that u could be talking about.' but this is just one pixel in a big picture. so if u take out one pixel then that's one little part of the big sexist picture that is society taken away. and then if another part was tackled, say for example, clothes advertised towards little boys are about being strong and brave and dinosaurs and cars, whereas, girls are about princesses and little baby animals and pink, then that's even more of a step towards a more equal society.

so what I'm saying is, unless ur going to start calling the mandem 'girls',  stop calling us 'guys' when we're in a group of ppl.