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Every younger sibling I know feels like they can’t do the older sibling’s “thing”. Older sibling is a soccer player? Can’t do that! Older sibling is a computer geek? Computers are now impossibly confusing. Older sibling likes star wars? Your only two options are star trek or to forgo science fiction altogether. I don’t know why this is the case but my guess is so that the younger sibling’s successes can’t be directly compared to the older one’s, which will probably be better because they’ve done it longer.
Now, I know Stan and Ford are twins and thus the same age. But I think something similar applies here: Stan couldn’t be a nerd because Ford got the title first. So he didn’t even try. Ford would always be smarter, so he embraced his other skills (of physical prowess and later scamming).
In order to rebuild the portal, Stan not only had to process his grief and guilt enough to do things, but also get over the feeling that math was “Ford’s thing” that he was “taking over”. He had to get over the “Ford’s the smart one” rhetoric. Stan gets way too little credit in the fandom for what he managed to pull off to bring Ford back, both mathematically/mentally, and emotionally.