They ask that questions also during student or work visa issuance - and even during the greencard (adjustment of status) and naturalization processes. They don't ask it every time you enter with these visas or statuses however.

But as a US citizen you will never be asked. And better yet if you are a dual citizen like me. Then you can travel on your other passport to the places the US finds to be undesirable.

I think green card and naturalization ask about travel in the last few years. But they don't ask about travel prior to that, and they don't ask about Iran specifically.

True - I think it was 10 (or was it 7?) year of travel history, but I'm sure Iran would raise some flags.