Below is an example sentence from the entry for the verb "alter" in Oxford Advanced Learner's Dictionary.( <= Is this a correct sentence? I am not confident with this. Excuse me for an irrelevant additional question.)
He had altered so much I scarcely recognized him.
I have been taught that "that" is omitted between "so much" and "I" in the sentence.
What makes it happen or what makes it possible to happen? I would like to get an answer focused on the reason of it.
Any personal opinions are also greatly helpful to me. I will appreciate any explanations from you. Thank you.