Border guards have the power to prevent you from entering the country if they believe your business is unlawful, and asking those questions is one of the ways they decide. We can question whether border guards ought to exist, but, given that they do, refusing to answer their questions seems like a ticket to a back room for hours.

You really never answered their questions on every entry?

If exercising my human rights is "a ticket to a back room for hours", then something is fundamentally broken in our society. You should try it; without doing so you actually have no data about the practical perimeter of your basic rights. This stuff isn't printed in the newspaper.

Yes, I really never answered their questions, except the ones about citizenship and nationality and place of birth—which I answered by presenting my passport.

"Business or pleasure?" is a vague, leading question designed to get you to volunteer as much information as possible. Sometimes I replied "no" or "yes" to that one, with an occasional "On advice of my attorney I decline to answer questions from police except in writing and via counsel" thrown in to break up the monotony.

Never talk to the police.

No. The USA can never deny entry to citizens (nor any other country). They may arrest you on the spot, but can't deny you entry.

Green card holders is a different story, they have no right to enter and are at the discretion of the authorities.