Why is it not necessary to use 很 (or other adverbs of degree) to link nouns and adjectives when you reduplicate the adjective, and instead use a 的 after the adjective?
For example:
你的女朋友高高的。
她们漂漂亮亮的。
很 as an adverb (very) only applies to adjective, never noun
A reduplicated adjective emphasizes itself similar to adding "quite" (颇/相当)
A reduplicated adjective cannot also use "very" because a reduplicated adjective emphasizes itself, similar to adding the adverb "quite".
Example:
他很强壮 = He is (very strong)
强强壮壮的他 = He who is (quite strong)
We can describe a man "very strong" or "quite strong" but not at same time, because they are different degree of superlative. 很 (very) is stronger than 颇 (quite). Is he very strong or quite strong?
"他(很)(强强壮壮的)" translated into English would be "He is very quite strong", which is not grammatical.
That reduplication pattern of adjectives is a cutie way to express quite. E. g. 她的眼睛大大的。他的个子高高的。
Well, 她的眼睛很大 and 他的个子很高 don't have that effect.
We don't say 她的眼睛很大大的, which means 很 doesn't work with this reduplication pattern.
There are two things going here, the reduplication, and a sentence structure that goes N + Adj + 的. I think of these sentences as:
你的女朋友高高的 = 他的女朋友 (是一个)高高的 (女朋友)
NB: The N + Adj + 的 structure is not limited to reduplicated adjectives.