Biden is the worst president since James Buchanan, who could also have claimed to be the most qualified president of his his time. A long time spent in government doesn’t necessarily provide the skills necessary to run the country.
John Quincy Adams was another example — lots of experience, and a better man and president than Biden or Buchanan, but not very skilled at politics. Hillary Clinton also claimed to be the most qualified candidate in history.
Senators don’t actually run anything, and nowadays most of them are routinely reelected, and don’t have to make difficult decisions.
Senators make bad presidents. Governors make better presidents because they have had to make hard decisions. Senators can be blowhard idiots and there is rarely any consequences.