I have put that fact in countless posts on this forum. The reason for that is Americans think the country is on the wrong track and sitting Senators are seen as part of the problem. As far as Duncan Hunter goes, no sitting Representative has been elected since Garfield and the story of how that came about is great story in American political history.
It’s not that Senators and Representatives are viewed as part of the problem, it’s because they are legislators.
Legislation is built upon the premise of compromise, and most people want to see someone of leadership more than someone who can get things done in a smoky back room.
That is why the vast majority of Presidents have either been Governors, Vice Presidents, or high ranking Generals. Those positions deal with leadership.
Sure you have the odd Garfield or Lincoln, but it is just very uncommon for a non-Governor/former VP to get elected.