It would sound absurd when you modify superlatives with adverbs such as very/extremely/much, etc.
I am the extremely smartest guy in the world.
It doesn't work as superlative means:
The highest extent or degree of something.
I think the same way of thinking should be applied to comparatives. When you use comparatives, you have a certain extent or degree in your mind higher or lower, albeit abstract, and you want to express it.
Comparitive is defined in Merriam-Webster as:
of or relating to the form of an adjective or adverb that is used to
indicate more of a particular quality
You want to express "more of a particular quality" and adding an adverb such as extremely can be a duplicate of comparatives.
That's why you don't use the adverb very to modify comparatives.
There are exceptional adverbs that are allowed by definition to modify comparatives such as:
Still, even, much, far, a lot, a way (colloquial)
Still has the following definition in Wiktionary:
(degree) To an even greater degree. Used to modify comparative
adjectives or adverbs.
Those other examples have the same definition and usage examples as still. Extremely doesn't.
Note: You can google extremely + comparative and get some hits. I don't think people would be unable to understand when they are uttered. .