Tim Cook couldn’t quit like Tesla’s Elon Musk and Disney’s Bob Iger because Cook never joined.
From the New York Times Risky Business for C.E.O.s: An Invitation From Trump:
[In December] the Blackstone Group’s chief executive, Stephen A. Schwarzman, announced the formation of the President’s Strategic and Policy Forum, whose members would “meet with the president frequently to share their specific experience and knowledge,” as Mr. Schwarzman put it at the time.
Then, executives were clamoring to make the cut, even those who had been cool or in some cases hostile to Mr. Trump’s candidacy. Well over 100 chief executives approached Mr. Schwarzman and the Trump transition team angling to be included…
The final list of 16 included such business luminaries as Jack Welch, the former chief executive of General Electric; Jamie Dimon of JPMorgan Chase; Bob Iger of Disney; Mary Barra of General Motors; and Virginia Rometty of IBM.
Soon after, Elon Musk of Tesla Motors, Travis Kalanick of Uber and Indra Nooyi of PepsiCo were added, buttressing the group’s expertise in technology and consumer products…
Only two people declined, citing time pressures: Apple’s chief executive, Tim Cook, and the Microsoft co-founder Bill Gates. (boldface mine)