"I am extremely smarter than you."

Solution 1:

The statement is grammatically acceptable. (You can change "extremely"for "much" or "slightly"and see how it works ).It's the choice of an adverb that makes the sentence strange.

Solution 2:

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. .