I serve at the pleasure of the president

During a congressional hearing on June 12, the Secretary of Defense, Pete Hegseth, used the phrase, “I serve at the pleasure of the president,” to avoid answering a question.
Why? He wants to keep his job.
The phrase, ”I serve at the pleasure of the president,” means that the President of the United States appointed you, and therefore can dismiss and replace you in any moment, for any reason he or she chooses, which can include the simple (and often petty) reason that they don’t like something you said or did.
And just like that, you’re kicked out of the club.
Hegseth also used the phrase on June 18, at a congressional hearing, when referring to the firing of the director of the National Security Agency, General Haugh.
“I don’t discuss who I talk about anything with, but ultimately, this is my decision and he serves at the pleasure of the president and that’s why he’s no longer there,” Hegseth said.

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Since all of the president’s appointees can use the statement “I serve at the pleasure of the president,” to refuse to answer a question or accept accountability for a mistake, then why do we bother with Congressional hearings at all?
Seriously.
They are basically pleading the fifth.
Why do we bother with Congressional hearings when presidential appointees don’t have to answer the questions?