It is presented in threes, probably to make the reader absorb and remember it more effectively. Steinbeck may have meant to "sear" his words into the brains of the readers.
For the record, I wasn't sure how to answer, so I looked up "Tricolon Rhetoric" and found this website:
I read the sentence and I must agree that Steinbeck wished to make his audience remember that man "grows beyond his work, walks up the stairs of his concepts, emerges ahead of his accomplishments" (pg 192-193).
it also has the intended effect of placing emphasis so that the reader focuses on the 'verb' structure.