Why is the traditional character for 你 also 你 and not 儞 when the traditional character for 尔 is 爾?
in the book 類篇　卷二十二 (page 87 of the liked pdf), there was an entry:

伲儞你乃倚切汝也或作儞你你又乃里切文三重音一

that, 伲　儞　你 ・[these three characters are pronounced as] 乃倚切・[have the meaning of] 汝 (you)
the book “類篇” is edited between 1039-1066, in 宋 dynasty.
i would say that 儞 / 你 existed in the past, just not commonly used as 汝
have fun :)